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 5e90dfc54f864651fd98087c6e1f1cbce203b20c)