socket: use INVALID_SOCKET
* Replace "fd = -1" with "fd = INVALID_SOCKET"
* Replace "fd < 0" with "fd == INVALID_SOCKET": SOCKET_T is unsigned on Windows
Bug found by Pavel Belikov ("Fragment N1"):
http://www.viva64.com/en/b/0414/#ID0ECDAE V
Victor Stinner committed
524714eeda70de01046e3b4736516f41d7d11010
Parent: 0cec877