CVE-2019-12436 dsdb/paged_results: ignore successful results without messages

CVE-2019-12436 dsdb/paged_results: ignore successful results without messages

So that we don't dereference result->msgs[0] when it doesn't exist.
This can happen when the object has changed in such a way that it no
longer matches the original search query.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13951

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Autobuild-User(master): Karolin Seeger <kseeger@samba.org>
Autobuild-Date(master): Wed Jun 19 08:16:39 UTC 2019 on sn-devel-184