ivi-layout: clear on_layer when layer_destroy

On_layer of ivi_layout_surface should be NULL in the layer_destroy.
Previous code might access to removed ivilayer and cause SEGV.

Signed-off-by: Wataru Natsume <wataru_natsume@xddp.denso.co.jp>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>

ivi-shell/ivi-layout.c

@@ -914,6 +914,7 @@ clear_surface_order_list(struct ivi_layout_layer *ivilayer)
 			      &ivilayer->order.surface_list, order.link) {
 		wl_list_remove(&surface_link->order.link);
 		wl_list_init(&surface_link->order.link);
+		surface_link->on_layer = NULL;
 	}
 }