GtkAssistant: Fix criticals on dispose

After the recent change thta introduced boxes between the pages
and the notebook, we were no longer careful enough when disposing
the assistant. Fix that up.

gtk/gtkassistant.c

@@ -1472,7 +1472,9 @@ gtk_assistant_remove (GtkContainer *container,
 
   /* Forward this removal to the content notebook */
   box = gtk_widget_get_parent (page);
-  if (gtk_widget_get_parent (box) == assistant->priv->content)
+  if (GTK_IS_BOX (box) &&
+      assistant->priv->content != NULL &&
+      gtk_widget_get_parent (box) == assistant->priv->content)
     {
       container = (GtkContainer *) assistant->priv->content;
       gtk_container_remove (container, box);