app: do not update a finalized dialog.

When the about dialog is destroyed, the weak link will be nullified.
Don't do anything then in our handler.

(cherry picked from commit cc9a15de97)

app/dialogs/about-dialog.c

@@ -774,6 +774,9 @@ about_dialog_last_release_changed (GimpCoreConfig   *config,
   g_signal_handlers_disconnect_by_func (config,
                                         (GCallback) about_dialog_last_release_changed,
                                         dialog);
+  if (! dialog->dialog)
+    return;
+
   if (dialog->update_frame)
     {
       gtk_widget_destroy (dialog->update_frame);