Fix divide by zero when autosave uses real time and autosave both disabled

See: #666

src/openttd.cpp

@@ -2236,7 +2236,7 @@ void StateGameLoop()
 			_state_ticks++;   // This must update in lock-step with _tick_skip_counter, such that _state_ticks_offset doesn't need to be changed.
 		}
 
-		if (!(_game_mode == GM_MENU || _game_mode == GM_BOOTSTRAP) && !_settings_client.gui.autosave_realtime &&
+		if (!(_game_mode == GM_MENU || _game_mode == GM_BOOTSTRAP) && !_settings_client.gui.autosave_realtime && _settings_client.gui.autosave_interval != 0 &&
 				(_state_ticks.base() % (_settings_client.gui.autosave_interval * (_settings_game.economy.tick_rate == TRM_MODERN ? (60000 / 27) : (60000 / 30)))) == 0) {
 			_do_autosave = true;
 			_check_special_modes = true;