Commit message: remove strncpy usage as it's effectively useless, replace with an assertion since fn is only used internally (fix #1426)
CVE: CVE-2018-11593
CWE: CWE-787
Vulnerability description: Espruino before 1.99 allows attackers to cause a denial of service (application crash) and potential Information Disclosure with a user crafted input file via a Buffer Overflow during syntax parsing because strncpy is misused in jslex.c.

Project: Espruino
File: jslex.c

void jslTokenAsString(int token, char *str, size_t len) {
  // see JS_ERROR_TOKEN_BUF_SIZE
  if (token>32 && token<128) {
    assert(len>=4);
    str[0] = '\'';
    str[1] = (char)token;
    str[2] = '\'';
    str[3] = 0;
    return;
  }

  switch (token) {
  case LEX_EOF : strncpy(str, "EOF", len); return;
  case LEX_ID : strncpy(str, "ID", len); return;
  case LEX_INT : strncpy(str, "INT", len); return;
  case LEX_FLOAT : strncpy(str, "FLOAT", len); return;
  case LEX_STR : strncpy(str, "STRING", len); return;
  case LEX_UNFINISHED_STR : strncpy(str, "UNFINISHED STRING", len); return;
  case LEX_TEMPLATE_LITERAL : strncpy(str, "TEMPLATE LITERAL", len); return;
  case LEX_UNFINISHED_TEMPLATE_LITERAL : strncpy(str, "UNFINISHED TEMPLATE LITERAL", len); return;
  case LEX_REGEX : strncpy(str, "REGEX", len); return;
  case LEX_UNFINISHED_REGEX : strncpy(str, "UNFINISHED REGEX", len); return;
  case LEX_UNFINISHED_COMMENT : strncpy(str, "UNFINISHED COMMENT", len); return;
  }
  if (token>=_LEX_OPERATOR_START && token<_LEX_R_LIST_END) {
    const char tokenNames[] =
        /* LEX_EQUAL      :   */ "==\0"
        /* LEX_TYPEEQUAL  :   */ "===\0"
        /* LEX_NEQUAL     :   */ "!=\0"
        /* LEX_NTYPEEQUAL :   */ "!==\0"
        /* LEX_LEQUAL    :    */ "<=\0"
        /* LEX_LSHIFT     :   */ "<<\0"
        /* LEX_LSHIFTEQUAL :  */ "<<=\0"
        /* LEX_GEQUAL      :  */ ">=\0"
        /* LEX_RSHIFT      :  */ ">>\0"
        /* LEX_RSHIFTUNSIGNED */ ">>>\0"
        /* LEX_RSHIFTEQUAL :  */ ">>=\0"
        /* LEX_RSHIFTUNSIGNEDEQUAL */ ">>>=\0"
        /* LEX_PLUSEQUAL   :  */ "+=\0"
        /* LEX_MINUSEQUAL  :  */ "-=\0"
        /* LEX_PLUSPLUS :     */ "++\0"
        /* LEX_MINUSMINUS     */ "--\0"
        /* LEX_MULEQUAL :     */ "*=\0"
        /* LEX_DIVEQUAL :     */ "/=\0"
        /* LEX_MODEQUAL :     */ "%=\0"
        /* LEX_ANDEQUAL :     */ "&=\0"
        /* LEX_ANDAND :       */ "&&\0"
        /* LEX_OREQUAL :      */ "|=\0"
        /* LEX_OROR :         */ "||\0"
        /* LEX_XOREQUAL :     */ "^=\0"
        /* LEX_ARROW_FUNCTION */ "=>\0"

        // reserved words
        /*LEX_R_IF :       */ "if\0"
        /*LEX_R_ELSE :     */ "else\0"
        /*LEX_R_DO :       */ "do\0"
        /*LEX_R_WHILE :    */ "while\0"
        /*LEX_R_FOR :      */ "for\0"
        /*LEX_R_BREAK :    */ "return\0"
        /*LEX_R_CONTINUE   */ "continue\0"
        /*LEX_R_FUNCTION   */ "function\0"
        /*LEX_R_RETURN     */ "return\0"
        /*LEX_R_VAR :      */ "var\0"
        /*LEX_R_LET :      */ "let\0"
        /*LEX_R_CONST :    */ "const\0"
        /*LEX_R_THIS :     */ "this\0"
        /*LEX_R_THROW :    */ "throw\0"
        /*LEX_R_TRY :      */ "try\0"
        /*LEX_R_CATCH :    */ "catch\0"
        /*LEX_R_FINALLY :  */ "finally\0"
        /*LEX_R_TRUE :     */ "true\0"
        /*LEX_R_FALSE :    */ "false\0"
        /*LEX_R_NULL :     */ "null\0"
        /*LEX_R_UNDEFINED  */ "undefined\0"
        /*LEX_R_NEW :      */ "new\0"
        /*LEX_R_IN :       */ "in\0"
        /*LEX_R_INSTANCEOF */ "instanceof\0"
        /*LEX_R_SWITCH     */ "switch\0"
        /*LEX_R_CASE       */ "case\0"
        /*LEX_R_DEFAULT    */ "default\0"
        /*LEX_R_DELETE     */ "delete\0"
        /*LEX_R_TYPEOF :   */ "typeof\0"
        /*LEX_R_VOID :     */ "void\0"
        /*LEX_R_DEBUGGER : */ "debugger\0"
        /*LEX_R_CLASS :    */ "class\0"
        /*LEX_R_EXTENDS :  */ "extends\0"
        /*LEX_R_SUPER :  */   "super\0"
        /*LEX_R_STATIC :   */ "static\0"
        ;
    unsigned int p = 0;
    int n = token-_LEX_OPERATOR_START;
    while (n>0 && p<sizeof(tokenNames)) {
      while (tokenNames[p] && p<sizeof(tokenNames)) p++;
      p++; // skip the zero
      n--; // next token
    }
    assert(n==0);
    strncpy(str, &tokenNames[p], len);
    return;
  }

  assert(len>=10);
  espruino_snprintf(str, len, "?[%d]", token);
}