libradius: Fix input validation bugs

Approved by: so
Security: FreeBSD-SA-21:12.libradius
Security: CVE-2021-29629
Sponsored by: The FreeBSD Foundation

(cherry picked from commit 8d5c7813061dfa0b187500dfe3aeea7a28181c13)
(cherry picked from commit 83280d17fccff2db7d79c7f38e80ec29078ef35e)