tipc: add NULL pointer check

skb somehow dequeued out of inputq before processing, it causes to
NULL pointer and kernel crashed.

Add checking skb valid before using.

Fixes: c55c8edafa ("tipc: smooth change between replicast and broadcast")
Reported-by: Tuong Lien Tong <tuong.t.lien@dektech.com.au>
Acked-by: Ying Xue <ying.xue@windriver.com>
Signed-off-by: Hoang Le <hoang.h.le@dektech.com.au>
Acked-by: Jon Maloy <jon.maloy@ericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/tipc/bcast.c

@@ -769,6 +769,9 @@ void tipc_mcast_filter_msg(struct net *net, struct sk_buff_head *defq,
 	u32 node, port;
 
 	skb = skb_peek(inputq);
+	if (!skb)
+		return;
+
 	hdr = buf_msg(skb);
 
 	if (likely(!msg_is_syn(hdr) && skb_queue_empty(defq)))