os/windows/posix.c: Use INVALID_SOCKET instead of < 0

os/windows/posix.c: Use INVALID_SOCKET instead of < 0

From
https://docs.microsoft.com/en-us/windows/win32/winsock/socket-data-type-2:
"Because the SOCKET type is unsigned, compiling existing source code from,
for example, a UNIX environment may lead to compiler warnings about
signed/unsigned data type mismatches.

This means, for example, that checking for errors when the socket and
accept functions return should not be done by comparing the return value
with -1, or seeing if the value is negative (both common and legal
approaches in UNIX). Instead, an application should use the manifest
constant INVALID_SOCKET as defined in the Winsock2.h header file."

This patch fixes the following build error for the latest version of
the MinGW64 cross-compiler on Cygwin:

os/windows/posix.c:886:17: error: comparison of unsigned expression < 0
  is always false

Signed-off-by: Bart Van Assche <bvanassche@acm.org>