pkt_sched: Fix oops in htb_delete.

Recent changes introduced a bug in htb_delete(): cl->parent->children
counter update misses checking cl->parent for NULL, which is used for
root classes, so deleting them causes an oops.
Signed-off-by: Jarek Poplawski <jarkao2@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/sch_htb.c

@@ -1279,7 +1279,8 @@ static int htb_delete(struct Qdisc *sch, unsigned long arg)
 
 	/* delete from hash and active; remainder in destroy_class */
 	qdisc_class_hash_remove(&q->clhash, &cl->common);
-	cl->parent->children--;
+	if (cl->parent)
+		cl->parent->children--;
 
 	if (cl->prio_activity)
 		htb_deactivate(q, cl);