net: sched: matchall: verify that filter is not NULL in mall_walk()

Check that filter is not NULL before passing it to tcf_walker->fn()
callback. This can happen when mall_change() failed to offload filter to
hardware.

Fixes: ed76f5ed

 ("net: sched: protect filter_chain list with filter_chain_lock mutex")
Reported-by: Ido Schimmel <idosch@mellanox.com>
Tested-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Vlad Buslov <vladbu@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>