net: sched: atm: dont intepret cls results when asked to drop

If asked to drop a packet via TC_ACT_SHOT it is unsafe to assume
res.class contains a valid pointer
Fixes: b0188d4d ("[NET_SCHED]: sch_atm: Lindent")
Signed-off-by: NJamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/sched/sch_atm.c

@@ -393,10 +393,13 @@ static int atm_tc_enqueue(struct sk_buff *skb, struct Qdisc *sch,
 				result = tcf_classify(skb, NULL, fl, &res, true);
 				if (result < 0)
 					continue;
+				if (result == TC_ACT_SHOT)
+					goto done;
+
 				flow = (struct atm_flow_data *)res.class;
 				if (!flow)
 					flow = lookup_flow(sch, res.classid);
-				goto done;
+				goto drop;
 			}
 		}
 		flow = NULL;