CVE-2020-14383: s4/dns: do not crash when additional data not found

CVE-2020-14383: s4/dns: do not crash when additional data not found

Found by Francis Brosnan Blázquez <francis@aspl.es>.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14472
BUG: https://bugzilla.samba.org/show_bug.cgi?id=12795

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Jeremy Allison <jra@samba.org>
Autobuild-User(master): Douglas Bagnall <dbagnall@samba.org>
Autobuild-Date(master): Mon Aug 24 00:21:41 UTC 2020 on sn-devel-184

(based on commit df98e7db04c901259dd089e20cd557bdbdeaf379)