gdesktopappinfo: Unref GDBus call result GVariant if no callback is set

When launching URIs via dbus we may ignore the callback if one was not
provided, however in such case we were also leaking the return value for
the gdbus call.

Unref it properly.

Helps with: https://gitlab.gnome.org/GNOME/glib/-/issues/333

gio/gdesktopappinfo.c

@@ -3108,6 +3108,9 @@ launch_uris_with_dbus_signal_cb (GObject      *object,
 
   if (data->callback)
     data->callback (object, result, data->user_data);
+  else if (!g_task_had_error (G_TASK (result)))
+    g_variant_unref (g_dbus_connection_call_finish (G_DBUS_CONNECTION (object),
+                                                    result, NULL));
 
   launch_uris_with_dbus_data_free (data);
 }