Fix #8711: having gui_zoom lower than zoom_min causes crashes (#8835)

gui_zoom was never clamp'd between zoom_min/zoom_max.

zoom_min controls how zoomed-in we load sprites. For a value of 1,
no quad-sizes sprites are loaded. If gui_zoom would be 0, meaning
it wants quad-sized sprites to display, it was printing random
stuff to the screen, which could or could not result in crashes.

src/gfx.cpp

@@ -1893,7 +1893,9 @@ void UpdateGUIZoom()
 	if (_gui_zoom_cfg == ZOOM_LVL_CFG_AUTO) {
 		_gui_zoom = static_cast<ZoomLevel>(Clamp(VideoDriver::GetInstance()->GetSuggestedUIZoom(), _settings_client.gui.zoom_min, _settings_client.gui.zoom_max));
 	} else {
-		_gui_zoom = static_cast<ZoomLevel>(_gui_zoom_cfg);
+		_gui_zoom = static_cast<ZoomLevel>(Clamp(_gui_zoom_cfg, _settings_client.gui.zoom_min, _settings_client.gui.zoom_max));
+		/* Write the value back in case it was not between min/max. */
+		_gui_zoom_cfg = _gui_zoom;
 	}
 
 	/* Determine real font zoom to use. */