util: clear pointers on wl_list_remove()

Set the next and prev pointers of the removed list element to NULL. This
will catch programming errors that would use invalid list pointers,
double-remove for instance.

It also helps debugging, making it easy to see in gdb if an object is
not in a list.

Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>

src/wayland-util.c

@@ -49,6 +49,8 @@ wl_list_remove(struct wl_list *elm)
 {
 	elm->prev->next = elm->next;
 	elm->next->prev = elm->prev;
+	elm->next = NULL;
+	elm->prev = NULL;
 }
 
 WL_EXPORT int