dfs_server: Use dsdb_search_one to catch 0 results as well as NO_SUCH_OBJECT errors

dfs_server: Use dsdb_search_one to catch 0 results as well as NO_SUCH_OBJECT errors

This ensures we do not de-reference an invalid rs->msgs pointer if the
pointed-to object was not objectclass=computer

Andrew Bartlett

Bug: https://bugzilla.samba.org/show_bug.cgi?id=10052

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>
Autobuild-User(master): Stefan Metzmacher <metze@samba.org>
Autobuild-Date(master): Fri Aug  2 13:11:20 CEST 2013 on sn-devel-104