net: sched: potential NULL dereference in tcf_block_find()

The error code isn't set on this path so it would result in returning
ERR_PTR(0) and a NULL dereference in the caller.

Fixes: 18d3eefb ("net: sched: refactor tcf_block_find() into standalone functions")
Signed-off-by: NDan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/sched/cls_api.c

@@ -1309,8 +1309,10 @@ static struct tcf_block *tcf_block_find(struct net *net, struct Qdisc **q,
 		goto errout_qdisc;
 
 	block = __tcf_block_find(net, *q, *cl, ifindex, block_index, extack);
-	if (IS_ERR(block))
+	if (IS_ERR(block)) {
+		err = PTR_ERR(block);
 		goto errout_qdisc;
+	}
 
 	return block;