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

@@ -760,20 +760,40 @@ bool NetworkClientConnectGame(NetworkAddress &address, CompanyID join_as, const
 	if (!_network_available) return false;
 	if (!NetworkValidateClientName()) return false;
 
-	strecpy(_settings_client.network.last_joined, address.GetAddressAsString(false).c_str(), lastof(_settings_client.network.last_joined));
-
+	_network_join.address = address;
 	_network_join.company = join_as;
 	_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;
+	}
+	return true;
+}
+
+/**
+ * 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();
 
+	strecpy(_settings_client.network.last_joined, _network_join.address.GetAddressAsString(false).c_str(), lastof(_settings_client.network.last_joined));
 	_network_join_status = NETWORK_JOIN_STATUS_CONNECTING;
 	ShowJoinStatusWindow();
 
-	new TCPClientConnecter(address);
-	return true;
+	new TCPClientConnecter(_network_join.address);
 }
 
 static void NetworkInitGameInfo()

src/network/network_client.h

@@ -115,6 +115,7 @@ void NetworkClientSetCompanyPassword(const char *password);
 /** Information required to join a server. */
 struct NetworkJoinInfo {
 	NetworkJoinInfo() : company(COMPANY_SPECTATOR), server_password(nullptr), company_password(nullptr) {}
+	NetworkAddress address;       ///< The address of the server to join.
 	CompanyID company;            ///< The company to join.
 	const char *server_password;  ///< The password of the server to join.
 	const char *company_password; ///< The password of the company to join.

src/network/network_func.h

@@ -52,6 +52,7 @@ void NetworkPopulateCompanyStats(NetworkCompanyStats *stats);
 void NetworkUpdateClientInfo(ClientID client_id);
 void NetworkClientsToSpectators(CompanyID cid);
 bool NetworkClientConnectGame(const std::string &connection_string, CompanyID default_company, 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

@@ -1068,6 +1068,11 @@ 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 */