Commit message: Avoid null pointer exception due to NULL return value from strchr.

This fixes #793.
CVE: CVE-2019-13114
CWE: CWE-476
Vulnerability description: http.c in Exiv2 through 0.27.1 allows a malicious http server to cause a denial of service (crash due to a NULL pointer dereference) by returning a crafted response that lacks a space character.

Project: exiv2
File: http.cpp

int Exiv2::http(Exiv2::Dictionary& request,Exiv2::Dictionary& response,std::string& errors)
{
    if ( !request.count("verb")   ) request["verb"   ] = "GET";
    if ( !request.count("header") ) request["header" ] = ""   ;
    if ( !request.count("version")) request["version"] = "1.0";
    if ( !request.count("port")   ) request["port"   ] = ""   ;

    std::string file;
    errors     = "";
    int result = 0;

    ////////////////////////////////////
    // Windows specific code
#ifdef WIN32
    WSADATA wsaData;
    WSAStartup(MAKEWORD(2,2), &wsaData);
#endif

    const char* servername = request["server" ].c_str();
    const char* page       = request["page"   ].c_str();
    const char* verb       = request["verb"   ].c_str();
    const char* header     = request["header" ].c_str();
    const char* version    = request["version"].c_str();
    const char* port       = request["port"   ].c_str();

    const char* servername_p = servername;
    const char* port_p       = port      ;
    std::string url = std::string("http://") + request["server"] + request["page"];

    // parse and change server if using a proxy
    const char* PROXI  = "HTTP_PROXY";
    const char* proxi  = "http_proxy";
    const char* PROXY  = getenv(PROXI);
    const char* proxy  = getenv(proxi);
    bool        bProx  = PROXY || proxy;
    const char* prox   = bProx ? (proxy?proxy:PROXY):"";
    Exiv2::Uri  Proxy  =  Exiv2::Uri::Parse(prox);

    // find the dictionary of no_proxy servers
    const char* NO_PROXI = "NO_PROXY";
    const char* no_proxi = "no_proxy";
    const char* NO_PROXY = getenv(NO_PROXI);
    const char* no_proxy = getenv(no_proxi);
    bool        bNoProxy = NO_PROXY||no_proxy;
    std::string no_prox  = std::string(bNoProxy?(no_proxy?no_proxy:NO_PROXY):"");
    Exiv2::Dictionary noProxy= stringToDict(no_prox + ",localhost,127.0.0.1");

    // if the server is on the no_proxy list ... ignore the proxy!
    if ( noProxy.count(servername) ) bProx = false;

    if (  bProx ) {
        servername_p = Proxy.Host.c_str();
        port_p       = Proxy.Port.c_str();
        page         = url.c_str();
        std::string  p(proxy?proxi:PROXI);
    //  std::cerr << p << '=' << prox << " page = " << page << std::endl;
    }
    if ( !port  [0] ) port   = "80";
    if ( !port_p[0] ) port_p = "80";

    ////////////////////////////////////
    // open the socket
    int     sockfd = (int) socket(AF_INET , SOCK_STREAM,IPPROTO_TCP) ;
    if (    sockfd < 0 ) return error(errors, "unable to create socket\n",NULL,NULL,0) ;

    // connect the socket to the server
    int     server  = -1 ;

    // fill in the address
    struct  sockaddr_in serv_addr   ;
    int                 serv_len = sizeof(serv_addr);
    memset((char *)&serv_addr,0,serv_len);

    serv_addr.sin_addr.s_addr   = inet_addr(servername_p);
    serv_addr.sin_family        = AF_INET    ;
    serv_addr.sin_port          = htons(atoi(port_p));

    // convert unknown servername into IP address
    // http://publib.boulder.ibm.com/infocenter/iseries/v5r3/index.jsp?topic=/rzab6/rzab6uafinet.htm
    if (serv_addr.sin_addr.s_addr == (unsigned long)INADDR_NONE)
    {
        struct hostent* host = gethostbyname(servername_p);
        if ( !host )  return error(errors, "no such host", servername_p);
        memcpy(&serv_addr.sin_addr,host->h_addr,sizeof(serv_addr.sin_addr));
    }

    makeNonBlocking(sockfd) ;

    ////////////////////////////////////
    // and connect
    server = connect(sockfd, (const struct sockaddr *) &serv_addr, serv_len) ;
    if ( server == SOCKET_ERROR && WSAGetLastError() != WSAEWOULDBLOCK )
        return error(errors,"error - unable to connect to server = %s port = %s wsa_error = %d",servername_p,port_p,WSAGetLastError());

    char   buffer[32*1024+1];
    size_t buff_l= sizeof buffer - 1 ;

    ////////////////////////////////////
    // format the request
    int    n  = snprintf(buffer,buff_l,httpTemplate,verb,page,version,servername,header) ;
    buffer[n] = 0 ;
    response["requestheaders"]=std::string(buffer,n);


    ////////////////////////////////////
    // send the header (we'll have to wait for the connection by the non-blocking socket)
    while ( sleep_ >= 0 && send(sockfd,buffer,n,0) == SOCKET_ERROR /* && WSAGetLastError() == WSAENOTCONN */ ) {
        Sleep(snooze) ;
        sleep_ -= snooze ;
    }

    if ( sleep_ < 0 )
        return error(errors,"error - timeout connecting to server = %s port = %s wsa_error = %d",servername,port,WSAGetLastError());

    int    end   = 0         ; // write position in buffer
    bool   bSearching = true ; // looking for headers in the response
    int    status= 200       ; // assume happiness

    ////////////////////////////////////
    // read and process the response
    int err ;
    n=forgive(recv(sockfd,buffer,(int)buff_l,0),err) ;
    while ( n >= 0 && OK(status) ) {
        if ( n ) {
            end += n ;
            buffer[end] = 0 ;

            size_t body = 0         ; // start of body
            if ( bSearching ) {

                // search for the body
                for ( size_t b = 0 ; bSearching && b < lengthof(blankLines) ; b++ ) {
                    if ( strstr(buffer,blankLines[b]) ) {
                        bSearching = false ;
                        body   = (int) ( strstr(buffer,blankLines[b]) - buffer ) + strlen(blankLines[b]) ;
                        status = atoi(strchr(buffer,' ')) ;
                    }
                }

                // parse response headers
                char* h = buffer;
                char  C = ':' ;
                char  N = '\n';
                int   i = 0   ; // initial byte in buffer
                while(buffer[i] == N ) i++;
                h       = strchr(h+i,N)+1;
                response[""]=std::string(buffer+i).substr(0,h-buffer-2);
                result = atoi(strchr(buffer,' '));
                char* c = strchr(h,C);
                char* first_newline = strchr(h,N);
                while ( c && first_newline && c < first_newline && h < buffer+body ) {
                    std::string key(h);
                    std::string value(c+1);
                    key   = key.substr(0,c-h);
                    value = value.substr(0,first_newline-c-1);
                    response[key]=value;
                    h = first_newline+1;
                    c = strchr(h,C);
                    first_newline = strchr(h,N);
                }
            }

            // if the bufffer's full and we're still searching - give up!
            // this handles the possibility that there are no headers
            if ( bSearching && buff_l-end < 10 ) {
                bSearching = false ;
                body  = 0 ;
            }
            if ( !bSearching && OK(status) ) {
                flushBuffer(buffer,body,end,file);
            }
        }
        n=forgive(recv(sockfd,buffer+end,(int)(buff_l-end),0),err) ;
        if ( !n ) {
            Sleep(snooze) ;
            sleep_ -= snooze ;
            if ( sleep_ < 0 ) n = FINISH ;
        }
    }

    if ( n != FINISH || !OK(status) ) {
        snprintf(buffer,sizeof buffer,"wsa_error = %d,n = %d,sleep_ = %d status = %d"
                ,   WSAGetLastError()
                ,   n
                ,   sleep_
                ,   status
                ) ;
        error(errors,buffer,NULL,NULL,0) ;
    } else if ( bSearching && OK(status) ) {
        if ( end ) {
        //  we finished OK without finding headers, flush the buffer
            flushBuffer(buffer,0,end,file) ;
        } else {
            return error(errors,"error - no response from server = %s port = %s wsa_error = %d",servername,port,WSAGetLastError());
        }
    }

    ////////////////////////////////////
    // close sockets
    closesocket(server) ;
    closesocket(sockfd) ;
    response["body"]=file;
    return result;
}