samba-tool drs showrepl: do not crash if no dnsHostName found

samba-tool drs showrepl: do not crash if no dnsHostName found

This should not happen, but it does sometimes in an autobuild
environment. Rather than reporting this by crashing, we report it by
showing there is no DNS name.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13716

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Autobuild-User(master): Douglas Bagnall <dbagnall@samba.org>
Autobuild-Date(master): Fri Oct 12 15:27:07 CEST 2018 on sn-devel-144

(cherry picked from commit 2fc855e7d2458249ca6fc8ffdf1d7633ab84cc55)