winbind: fix crash in fill_domain_username_talloc() if specified username is NULL

Signed-off-by: Ralf Habacker <ralf.habacker@freenet.de>
Reviewed-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>

Autobuild-User(master): Jeremy Allison <jra@samba.org>
Autobuild-Date(master): Wed Jul  3 19:24:52 UTC 2019 on sn-devel-184