w32:estream: Use INVALID_SOCKET to check invalid socket.

* src/estream.c (IS_INVALID_SOCKET): New.
(func_sock_read, func_sock_write, func_sock_ioctl, func_sock_destroy):
Use IS_INVALID_SOCKET.

It is possible for some architectures that the value of extended -1
and ~0 is different in C. Better to avoid the issue in advance.

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>