pkt_sched: fix missing check for packet overrun in qdisc_dump_stab()

nla_nest_start() might return NULL, causing a NULL pointer dereference.
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/sch_api.c

@@ -417,6 +417,8 @@ static int qdisc_dump_stab(struct sk_buff *skb, struct qdisc_size_table *stab)
 	struct nlattr *nest;
 
 	nest = nla_nest_start(skb, TCA_STAB);
+	if (nest == NULL)
+		goto nla_put_failure;
 	NLA_PUT(skb, TCA_STAB_BASE, sizeof(stab->szopts), &stab->szopts);
 	nla_nest_end(skb, nest);