rec: Fix DNSSEC validation of DS denial from the negative cache

rec: Fix DNSSEC validation of DS denial from the negative cache

There is two reasons you can get a proper DS denial:
* Secure to insecure cut, and if we are getting a referral with a
DS denial, we know that we have to check that the NS bit is set
as described in section 8.9 of rfc5155
* No zone cut inside a secure zone, and then of course the NS is
not set

When we retrieve the DS denial from the negative cache with a
validation status of Indeterminate, most likely because validation
was not enabled during the query that landed it in the cache, we
don't have enough data to know which case we are looking at, so
let's just skip the NS check.