util: null-out the node's prev/next pointers in list_del()

Note: This is a candidate for the 9.0 branch.
Signed-off-by: Brian Paul <brianp@vmware.com>

src/gallium/auxiliary/util/u_double_list.h

@@ -82,6 +82,7 @@ static INLINE void list_del(struct list_head *item)
 {
     item->prev->next = item->next;
     item->next->prev = item->prev;
+    item->prev = item->next = NULL;
 }
 
 static INLINE void list_delinit(struct list_head *item)