dialog: Avoid possible use-after-free

When the dialogue's titlebar was replaced, we were still trying to
update the label we constructed but that was now destroyed.

https://bugzilla.gnome.org/show_bug.cgi?id=726492

gtk/gtkdialog.c

@@ -304,7 +304,7 @@ apply_use_header_bar (GtkDialog *dialog)
           label = gtk_label_new ("");
           gtk_style_context_add_class (gtk_widget_get_style_context (label), "title");
           gtk_box_set_center_widget (GTK_BOX (box), label);
-          g_signal_connect (dialog, "notify::title", G_CALLBACK (update_title), label);
+          g_signal_connect_object (dialog, "notify::title", G_CALLBACK (update_title), label, 0);
         }
 
       gtk_window_set_titlebar (GTK_WINDOW (dialog), box);