CVE-2019-19344 kcc dns scavenging: Fix use after free in dns_tombstone_records_zone

CVE-2019-19344 kcc dns scavenging: Fix use after free in dns_tombstone_records_zone

ldb_msg_add_empty reallocates the underlying element array, leaving
old_el pointing to freed memory.

This patch takes two defensive copies of the ldb message, and performs
the updates on them rather than the ldb messages in the result.

Bug: https://bugzilla.samba.org/show_bug.cgi?id=14050

Signed-off-by: Gary Lockyer <gary@catalyst.net.nz>
Autobuild-User(master): Karolin Seeger <kseeger@samba.org>
Autobuild-Date(master): Tue Jan 21 11:38:38 UTC 2020 on sn-devel-184