MFC r360967: libalias: validate packet lengths before accessing headers

admbugs: 956
Submitted by: ae
Reported by: Lucas Leong (@_wmliang_) of Trend Micro Zero Day Initiative
Reported by: Vishnu working with Trend Micro Zero Day Initiative
Approved by: so
Security: FreeBSD-SA-20:12.libalias
Security: CVE-2020-7454
Security: ZDI-CAN-10624, ZDI-CAN-10850