sasl: don't use uninitialized variable

base64_decode() used uninitialized variable when unable to allocate memory.

Reported-by: Fabian Freyer <fabian.freyer@physik.tu-berlin.de>
SCRAM-SHA-1 authentication mechanism support
md5: MD5Transform should be static
sha1: workaround for name collision with libcrypto

src/sha1.c is used only for internal purposes. So, just add prefix
xmpp_ to the SHA1 symbols.
XEP-0114: external component authentication

New API xmpp_connect_component() introduced.
XEP-0114: minor fixes to auth.c

 - src/sha1.c is used instead of openssl;
 - xmpp_alloc/free should be used instead of malloc/free;
 - coding style fixes.