update_settings.cpp: avoid possible segfault

fw_url is not initialized without defined update.url

src/gui/update_settings.cpp

@@ -174,6 +174,7 @@ int CUpdateSettings::initMenu()
 	}
 #endif
 	w_upsettings.addItem(fw_update_dir);
+	if (fw_url)
 		w_upsettings.addItem(fw_url);
 #if ENABLE_EXTUPDATE
 	w_upsettings.addItem(name_backup);