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_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.