net: tcp: remove BUG_ON from tcp_v4_err

if skb is NULL pointer, and the following access of skb's
skb_mstamp_ns will trigger panic, which is same as BUG_ON
Signed-off-by: NLi RongQing <lirongqing@baidu.com>
Signed-off-by: NEric Dumazet <edumazet@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/tcp_ipv4.c

@@ -542,7 +542,6 @@ int tcp_v4_err(struct sk_buff *icmp_skb, u32 info)
 		icsk->icsk_rto = inet_csk_rto_backoff(icsk, TCP_RTO_MAX);
 
 		skb = tcp_rtx_queue_head(sk);
-		BUG_ON(!skb);
 
 		tcp_mstamp_refresh(tp);
 		delta_us = (u32)(tp->tcp_mstamp - tcp_skb_timestamp_us(skb));