Bug 676940 - Group children prevent droping layer at bottom of stack

...and don't forget to return the drop_path from
gimp_container_tree_view_drop_status().

app/widgets/gimpcontainertreeview-dnd.c

@@ -161,6 +161,7 @@ gimp_container_tree_view_drop_status (GimpContainerTreeView    *tree_view,
                               &renderer,
                               -1);
 
+          drop_path     = gtk_tree_model_get_path (tree_view->model, &iter);
           dest_viewable = renderer->viewable;
           drop_pos      = GTK_TREE_VIEW_DROP_AFTER;