net: cls_bpf: fix NULL deref on offload filter removal

Commit 40119211 ("net: sched: refactor block offloads counter
usage") missed the fact that either new prog or old prog may be
NULL.

Fixes: 40119211 ("net: sched: refactor block offloads counter usage")
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>