Fix a page fault in ip6_setpktopt(), occurring when the pflog module is
loaded, and syncthing is started, which uses setsockopt(IPV6_PKGINFO).

This is because pflog interfaces do not normally have an IPv6 address,
causing the ND_IFINFO() macro to dereference a NULL pointer.

Reviewed by: ae
PR: 210943
MFC after: 3 days