fix incorrectly set AD flag for CNAME chains

Valid CNAME chains that ended in NODATA had AD flag set.
6 jobs for <span class="ref-name">ad-cname-chains</span> in 76 minutes and 11 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
canceled #72889
docker linux amd64
build:linux:amd64

passed #13171
docker linux amd64
build:linux:amd64

00:00:35

 
  Test
canceled #73103
docker linux amd64
deckard:linux:amd64

passed #13174
docker linux amd64 allowed to fail
respdiff:linux:amd64

01:16:11

passed #13172
docker linux amd64
test:linux:amd64

00:00:19

passed #13173
docker linux amd64
deckard:linux:amd64

00:00:29