net: sched: ensure n arg not empty before call bind_class

All bind_class callbacks are directly returned when n arg is empty.
Therefore, bind_class is invoked only when n arg is not empty.
Signed-off-by: Zhengchao Shao <shaozhengchao@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/sch_api.c

@@ -1915,7 +1915,7 @@ static int tcf_node_bind(struct tcf_proto *tp, void *n, struct tcf_walker *arg)
 {
 	struct tcf_bind_args *a = (void *)arg;
 
-	if (tp->ops->bind_class) {
+	if (n && tp->ops->bind_class) {
 		struct Qdisc *q = tcf_block_q(tp->chain->block);
 
 		sch_tree_lock(q);