netfilter: nfnetlink_osf: Fix a missing skb_header_pointer() NULL check

[ Upstream commit 5e024c325406470d1165a09c6feaf8ec897936be ]

Do not assume that the tcph->doff field is correct when parsing for TCP
options, skb_header_pointer() might fail to fetch these bits.

Fixes: 11eeef41 ("netfilter: passive OS fingerprint xtables match")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>