CLEANUP: http: silence a cppcheck warning in get_http_auth()

CLEANUP: http: silence a cppcheck warning in get_http_auth()

In issue #777, cppcheck wrongly assumes a useless null pointer check
in the expression below while it's obvious that in a 3G/1G split on
32-bit, len can become positive if p is NULL:

     p = memchr(ctx.value.ptr, ' ', ctx.value.len);
     len = p - ctx.value.ptr;
     if (!p || len <= 0)
           return 0;

In addition, on 64 bits you never know given that len is a 32-bit signed
int thus the sign of the result in case of a null p will always be the
opposite of the 32th bit of ctx.value.ptr. Admittedly the test is ugly.

Tim proposed this fix consisting in checking for p == ctx.value.ptr
instead when checking for first character only, which Ilya confirmed is
enough to shut cppcheck up. No backport is needed.