Fix #6598: Prevent invalid memory accesses when abandoning a join from within a network game

One could join a network game from within an already running network game. This would call a NetworkDisconnect, but keeps the UI alive. If, during that process the join is aborted, e.g. by cancelling on a password dialog, you would still be in your network game but also get shown the server list.
Solve all the underlying problems by falling back to the main UI when (re)connecting to a(nother) server.

src/network/network.cpp

@@ -670,13 +670,33 @@ void NetworkClientConnectGame(const char *hostname, uint16 port, CompanyID join_
 	_network_join_server_password = join_server_password;
 	_network_join_company_password = join_company_password;
 
+	if (_game_mode == GM_MENU) {
+		/* From the menu we can immediately continue with the actual join. */
+		NetworkClientJoinGame();
+	} else {
+		/* When already playing a game, first go back to the main menu. This
+		 * disconnects the user from the current game, meaning we can safely
+		 * load in the new. After all, there is little point in continueing to
+		 * play on a server if we are connecting to another one.
+		 */
+		_switch_mode = SM_JOIN_GAME;
+	}
+}
+
+/**
+ * Actually perform the joining to the server. Use #NetworkClientConnectGame
+ * when you want to connect to a specific server/company. This function
+ * assumes _network_join is already fully set up.
+ */
+void NetworkClientJoinGame()
+{
 	NetworkDisconnect();
 	NetworkInitialize();
 
 	_network_join_status = NETWORK_JOIN_STATUS_CONNECTING;
 	ShowJoinStatusWindow();
 
-	new TCPClientConnecter(NetworkAddress(hostname, port));
+	new TCPClientConnecter(NetworkAddress(_settings_client.network.last_host, _settings_client.network.last_port));
 }
 
 static void NetworkInitGameInfo()

src/network/network_func.h

@@ -49,6 +49,7 @@ void NetworkPopulateCompanyStats(NetworkCompanyStats *stats);
 void NetworkUpdateClientInfo(ClientID client_id);
 void NetworkClientsToSpectators(CompanyID cid);
 void NetworkClientConnectGame(const char *hostname, uint16 port, CompanyID join_as, const char *join_server_password = nullptr, const char *join_company_password = nullptr);
+void NetworkClientJoinGame();
 void NetworkClientRequestMove(CompanyID company, const char *pass = "");
 void NetworkClientSendRcon(const char *password, const char *command);
 void NetworkClientSendChat(NetworkAction action, DestType type, int dest, const char *msg, int64 data = 0);

src/openttd.cpp

@@ -1109,6 +1109,11 @@ void SwitchToMode(SwitchMode new_mode)
 			break;
 		}
 
+		case SM_JOIN_GAME: // Join a multiplayer game
+			LoadIntroGame();
+			NetworkClientJoinGame();
+			break;
+
 		case SM_MENU: // Switch to game intro menu
 			LoadIntroGame();
 			if (BaseSounds::ini_set.empty() && BaseSounds::GetUsedSet()->fallback && SoundDriver::GetInstance()->HasOutput()) {

src/openttd.h

@@ -36,6 +36,7 @@ enum SwitchMode {
 	SM_START_HEIGHTMAP,   ///< Load a heightmap and start a new game from it.
 	SM_LOAD_HEIGHTMAP,    ///< Load heightmap from scenario editor.
 	SM_RESTART_HEIGHTMAP, ///< Load a heightmap and start a new game from it with current settings.
+	SM_JOIN_GAME,         ///< Join a network game.
 };
 
 /** Display Options */