socket: Fix internal_select()

Bug found by Pavel Belikov ("Fragment N1"):
http://www.viva64.com/en/b/0414/#ID0ECDAE

Modules/socketmodule.c

@@ -648,7 +648,7 @@ internal_select(PySocketSockObject *s, int writing, _PyTime_t interval,
     assert(!(connect && !writing));
 
     /* Guard against closed socket */
-    if (s->sock_fd < 0)
+    if (s->sock_fd == INVALID_SOCKET)
         return 0;
 
     /* Prefer poll, if available, since you can poll() any fd