radlib: fix a memory leak in is_valid_request

Call HMAC_CTX_free if returning early from is_valid_request when
processing Message-Authenticator tags.

Reported by: Coverity
MFC after: 1 week
Fixes: 8d5c7813061d ("libradius: Fix input validation bugs")
Differential Revision: https://reviews.freebsd.org/D45488