main: fix memleak, when reading g_settings

src/main.c

@@ -1422,6 +1422,7 @@ activate(GtkApplication *app, gpointer data)
                 g_settings_set_string(settings, "postprocessor", setting_postproc);
                 printf("Initialized postprocessor to %s\n", setting_postproc);
         }
+	free(setting_postproc);
 
         // Find all postprocessors for the settings list
         mp_process_find_all_processors(setting_postprocessor_list);