Fix #4422: Save overwrite multiplayer

Make sure we reset gFirstTimeSave when we join a new game or exit to title screen.

src/network/network.cpp

@@ -1540,6 +1540,7 @@ void Network::Client_Handle_MAP(NetworkConnection& connection, NetworkPacket& pa
 			server_srand0_tick = 0;
 			// window_network_status_open("Loaded new map from network");
 			_desynchronised = false;
+			gFirstTimeSave = 1;
 
 			// Notify user he is now online and which shortcut key enables chat
 			network_chat_show_connected_message();