Prevent a segfault if GtkAppChooserDialog gets disposed early

If the async call is finished after the dialog is already disposed,
bad things happen. Prevent that by marking the dialog as dismissed
in this case.

gtk/gtkappchooserdialog.c

@@ -536,6 +536,8 @@ gtk_app_chooser_dialog_dispose (GObject *object)
   cancel_and_clear_cancellable (self);
   g_clear_object (&self->priv->online);
 
+  self->priv->dismissed = TRUE;
+
   G_OBJECT_CLASS (gtk_app_chooser_dialog_parent_class)->dispose (object);
 }