lib/resolve answer_finalize: don't SERVFAIL bogus +cd

lib/resolve answer_finalize: don't SERVFAIL bogus +cd

As kresd works now, typically we do not know whether these records are
bogus, as with +cd we do not attempt validation.  Still, it's possible
that we have those records in cache from an occasion without +cd, in
which case we know they're bogus and this regression happened.

The potential impact of this issue seems minimal.