Explicitly cast INVALID_SOCKET to (int) to address warnings on Windows.

Even though SOCKET is effectively declared as (void *) on Windows, it's
not actually a pointer, but an index within per-process table of
kernel objects. The table size is actually limited and its upper limit
is far below upper limit for signed 32-bit integer. This is what makes
cast in question possible.

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Tim Hudson <tjh@openssl.org>