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: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>