winbind: Fix CID 1398534 Dereference before null check

Make all query_user_list backends consistent.

Signed-off-by: Jeremy Allison <jra@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>

Autobuild-User(master): Andreas Schneider <asn@cryptomilk.org>
Autobuild-Date(master): Fri Jan 13 13:33:37 CET 2017 on sn-devel-144
