cdac046 dns.c: fix input validation fix
Due to a slight foobar typo, failing to de-reference a pointer, previous
fix not quite as complete as it should have been.
Improve CVE-2020-11750 fix
Signed-off-by: Kevin Darbyshire-Bryant <ldir@darbyshire-bryant.me.uk>
(cherry picked from commit 9f7c8ed0786be97eda879e5f6681994e4de53d74)