dnsserver: Stop dns_name_equal doing OOB read

dnsserver: Stop dns_name_equal doing OOB read

This has been the cause of a large number of flakey autobuilds. Every
now and again dns_name_equal would not be equal between two empty
strings, thus causing failures.

Signed-off-by: Garming Sam <garming@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
BUG: https://bugzilla.samba.org/show_bug.cgi?id=12813

Autobuild-User(master): Andrew Bartlett <abartlet@samba.org>
Autobuild-Date(master): Thu Jun  1 19:34:38 CEST 2017 on sn-devel-144