socket: Fix internal_select()

Bug found by Pavel Belikov ("Fragment N1"):
http://www.viva64.com/en/b/0414/#ID0ECDAE
diff --git a/Modules/socketmodule.c b/Modules/socketmodule.c
index 8f571a2..8e804af 100644
--- a/Modules/socketmodule.c
+++ b/Modules/socketmodule.c
@@ -648,7 +648,7 @@
     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