netfilter: nft_tunnel: fix null-attribute check

else we get null deref when one of the attributes is missing, both
must be non-null.

Reported-by:  <syzbot+76d0b80493ac881ff77b@syzkaller.appspotmail.com>
Fixes: aaecfdb5 ("netfilter: nf_tables: match on tunnel metadata")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>