dns_rdataset_addnoqname() could find unsigned NSEC/NSEC3

dns_rdataset_addnoqname() could find unsigned NSEC/NSEC3

The dns_rdatalist addnoqname() implementation searches for the first
NSEC or NSEC3 record in a message, then for the first RRSIG covering
that type in the same message.  Previously, if no RRSIG for the type was
found, the function accepted the unsigned record. Now, it will instead
continue searching until an NSEC or NSEC3 that does have a matching
signature is found.

When this function is called from validated() in resolver.c, a
non-success return code is now treated as an error instead of triggering
an assertion failure.

Fixes: isc-projects/bind9#5985