netfilter: nft_osf: add missing check for DREG attribute

syzbot reports just another NULL deref crash because of missing test
for presence of the attribute.

Reported-by:  <syzbot+cf23983d697c26c34f60@syzkaller.appspotmail.com>
Fixes:  b96af92d

 ("netfilter: nf_tables: implement Passive OS fingerprint module in nft_osf")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>