net: sched: fix possible null pointer deref in tcf_block_put

We need to check block for being null in both tcf_block_put and
tcf_block_put_ext.

Fixes: 343723dd ("net: sched: fix clsact init error path")
Reported-by: NPrashant Bhole <bhole_prashant_q7@lab.ntt.co.jp>
Signed-off-by: NJiri Pirko <jiri@mellanox.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/sched/cls_api.c

@@ -379,6 +379,8 @@ void tcf_block_put(struct tcf_block *block)
 {
 	struct tcf_block_ext_info ei = {0, };
 
+	if (!block)
+		return;
 	tcf_block_put_ext(block, block->q, &ei);
 }