net_sched: check NULL on error path in route4_change()

On error path in route4_change(), 'f' could be NULL,
so we should check NULL before calling tcf_exts_destroy().

Fixes: b9a24bb7 ("net_sched: properly handle failure case of tcf_exts_init()")
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Cc: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/cls_route.c

@@ -559,7 +559,8 @@ static int route4_change(struct net *net, struct sk_buff *in_skb,
 	return 0;
 
 errout:
-	tcf_exts_destroy(&f->exts);
+	if (f)
+		tcf_exts_destroy(&f->exts);
 	kfree(f);
 	return err;
 }