net: sched: Fix a possible null-pointer dereference in dequeue_func()

In dequeue_func(), there is an if statement on line 74 to check whether
skb is NULL:
    if (skb)

When skb is NULL, it is used on line 77:
    prefetch(&skb->end);

Thus, a possible null-pointer dereference may occur.

To fix this bug, skb->end is used when skb is not NULL.

This bug is found by a static analysis tool STCheck written by us.

Fixes: 76e3cc12 ("codel: Controlled Delay AQM")
Signed-off-by: Jia-Ju Bai <baijiaju1990@gmail.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/sch_codel.c

@@ -71,10 +71,10 @@ static struct sk_buff *dequeue_func(struct codel_vars *vars, void *ctx)
 	struct Qdisc *sch = ctx;
 	struct sk_buff *skb = __qdisc_dequeue_head(&sch->q);
 
-	if (skb)
+	if (skb) {
 		sch->qstats.backlog -= qdisc_pkt_len(skb);
-
-	prefetch(&skb->end); /* we'll need skb_shinfo() */
+		prefetch(&skb->end); /* we'll need skb_shinfo() */
+	}
 	return skb;
 }