caif: added check for potential null return

Add check on NULL return from caif_get().

Signed-off-by: Kim Lilliestierna <Kim.xx.Lilliestierna@stericsson.com>
Signed-off-by: Sjur Brændeland <sjur.brandeland@stericssion.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/caif/caif_dev.c

@@ -128,6 +128,11 @@ void caif_flow_cb(struct sk_buff *skb)
 
 	rcu_read_lock();
 	caifd = caif_get(skb->dev);
+
+	WARN_ON(caifd == NULL);
+	if (caifd == NULL)
+		return;
+
 	caifd_hold(caifd);
 	rcu_read_unlock();