Assistant: Fix page rmoval

We have to remove the page itself from the intermediate box
first, before removing the box from the notebook. Otherwise,
reffing the page to keep it alive is ineffective: the box
gets destroyed, and that destruction recurses over the page.

This fixes the problem in
https://bugzilla.gnome.org/show_bug.cgi?id=756385

gtk/gtkassistant.c

@@ -1472,8 +1472,8 @@ gtk_assistant_remove (GtkContainer *container,
       assistant->priv->content != NULL &&
       gtk_widget_get_parent (box) == assistant->priv->content)
     {
-      container = (GtkContainer *) assistant->priv->content;
-      gtk_container_remove (container, box);
+      gtk_container_remove (GTK_CONTAINER (box), page);
+      gtk_container_remove (GTK_CONTAINER (assistant->priv->content), box);
     }
 }