s3:auth_winbind: ignore a missing winbindd as NT4 PDC/BDC without trusts

s3:auth_winbind: ignore a missing winbindd as NT4 PDC/BDC without trusts

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13722

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Alexander Bokovoy <ab@samba.org>
Autobuild-User(master): Alexander Bokovoy <ab@samba.org>
Autobuild-Date(master): Thu Dec 20 12:15:09 CET 2018 on sn-devel-144

(cherry picked from commit 63dc60767eb13d8fc09ed4bc44faa538581b18f1)

Autobuild-User(v4-9-test): Karolin Seeger <kseeger@samba.org>
Autobuild-Date(v4-9-test): Wed Jan  9 14:01:30 CET 2019 on sn-devel-144