Codechange: ensure OnConnect() always gets called with a valid socket (#9729)

This should already be the case, but now assert()s will tell us
if this isn't.

src/network/core/tcp.cpp

@@ -217,6 +217,8 @@ Packet *NetworkTCPSocketHandler::ReceivePacket()
  */
 bool NetworkTCPSocketHandler::CanSendReceive()
 {
+	assert(this->sock != INVALID_SOCKET);
+
 	fd_set read_fd, write_fd;
 	struct timeval tv;