netfilter: nfnetlink_osf: fix using plain integer as NULL warning

Fixes the following sparse warning:

net/netfilter/nfnetlink_osf.c:274:24: warning:
 Using plain integer as NULL pointer

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>