net_sched: check NULL in tcf_block_put()

net_sched: check NULL in tcf_block_put()

Callers of tcf_block_put() could pass NULL so
we can't use block->q before checking if block is
NULL or not.

tcf_block_put_ext() callers are fine, it is always
non-NULL.

Fixes: 8c4083b30e56 ("net: sched: add block bind/unbind notif. and extended block_get/put")
Reported-by: Dave Taht <[email protected]>
Cc: Jiri Pirko <[email protected]>
Signed-off-by: Cong Wang <[email protected]>
Reviewed-by: Jiri Pirko <[email protected]>
Signed-off-by: David S. Miller <[email protected]>