net: remove BUG_ON from __pskb_pull_tail

if list is NULL pointer, and the following access of list
will trigger panic, which is same as BUG_ON
Signed-off-by: Li RongQing <lirongqing@baidu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/skbuff.c

@@ -1925,8 +1925,6 @@ void *__pskb_pull_tail(struct sk_buff *skb, int delta)
 		struct sk_buff *insp = NULL;
 
 		do {
-			BUG_ON(!list);
-
 			if (list->len <= eat) {
 				/* Eaten as whole. */
 				eat -= list->len;