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_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);