pf: IPv6 fragments with malformed extension headers could be erroneously passed by pf or cause a panic

We mistakenly used the extoff value from the last packet to patch the
next_header field. If a malicious host sends a chain of fragmented packets
where the first packet and the final packet have different lengths or number of
extension headers we'd patch the next_header at the wrong offset.
This can potentially lead to panics or rule bypasses.

Security: CVE-2019-5597
Obtained from: OpenBSD
Reported by: Corentin Bayet, Nicolas Collignon, Luca Moro at Synacktiv