ENet wrapper properly detect disconnect on poll

Now PacketPeerUDP.get_available_packet_count() return -1 if the socket
is in error state.

drivers/windows/packet_peer_udp_winsock.cpp

@@ -42,7 +42,7 @@ int PacketPeerUDPWinsock::get_available_packet_count() const {
 
 	Error err = const_cast<PacketPeerUDPWinsock *>(this)->_poll(false);
 	if (err != OK)
-		return 0;
+		return -1;
 
 	return queue_count;
 }