Fix #10368, Fix 994bf19: server restarting game caused clients to hit assertion

Upon closing the EndGameWindow, triggered from UnInitWindowSystem, the
HighScoreWindow would be opened and _z_windows would not be empty.

src/highscore_gui.cpp

@@ -126,7 +126,7 @@ struct EndGameWindow : EndGameHighScoreBaseWindow {
 	void Close() override
 	{
 		if (!_networking) Command<CMD_PAUSE>::Post(PM_PAUSED_NORMAL, false); // unpause
-		ShowHighscoreTable(this->window_number, this->rank);
+		if (_game_mode != GM_MENU) ShowHighscoreTable(this->window_number, this->rank);
 		this->EndGameHighScoreBaseWindow::Close();
 	}