user-accounts: Don't leak a dialog

https://bugzilla.gnome.org/show_bug.cgi?id=708286

panels/user-accounts/um-account-dialog.c

@@ -1581,6 +1581,8 @@ um_account_dialog_dispose (GObject *obj)
                 self->enterprise_domain_timeout_id = 0;
         }
 
+        g_clear_pointer (&self->join_dialog, gtk_widget_destroy);
+
         G_OBJECT_CLASS (um_account_dialog_parent_class)->dispose (obj);
 }