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: Prashant Bhole <bhole_prashant_q7@lab.ntt.co.jp>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>