taprio: add null check on sched_nest to avoid potential null pointer dereference

The call to nla_nest_start_noflag can return a null pointer and currently
this is not being checked and this can lead to a null pointer dereference
when the null pointer sched_nest is passed to function nla_nest_end. Fix
this by adding in a null pointer check.

Addresses-Coverity: ("Dereference null return value")
Fixes: a3d43c0d ("taprio: Add support adding an admin schedule")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/sch_taprio.c

@@ -1087,6 +1087,8 @@ static int taprio_dump(struct Qdisc *sch, struct sk_buff *skb)
 		goto done;
 
 	sched_nest = nla_nest_start_noflag(skb, TCA_TAPRIO_ATTR_ADMIN_SCHED);
+	if (!sched_nest)
+		goto options_error;
 
 	if (dump_schedule(skb, admin))
 		goto admin_error;