libradius: fix no SSL build

int alen is only used with SSL.

(cherry picked from commit 48b11217bf7e605e1c386e316cd6e935aceb4927)

libradius: fix WITHOUT_OPENSSL build

int alen is used only with SSL.

Reported by: Michael Dexter, Build Option Survey
MFC after: 3 days
Fixes: 8d5c7813061d ("libradius: Fix input validation bugs")
Sponsored by: The FreeBSD Foundation

(cherry picked from commit a901f2af587f9cb068e2fca6b62f324bdde471d8)