fix warning: comparison of unsigned expression < 0 is always false [-Wtautological-compare]

Don't check for a negative parameter count, because not only will it
never happen, it doesn't make any sense either.

Invalid sockets (as returned by socket(2)) are always exactly -1 (not
just negative as negative file descriptors are technically not
prohibited by POSIX) on POSIX systems. Since we store them in SOCKET
(unsigned int), however, that really is ~0U (or MAX_UINT) which happens
to be what INVALID_SOCKET is already defined to, so an additional check
for being negative is not only unnecessary (unsigned integers aren't
*ever* negative) its redundant as well (the INVALID_SOCKET comparison is
enough).

Signed-off-by: Giel van Schijndel <me@mortis.eu>