s3: smbd: Ensure get_real_filename() copes with empty pathnames.

s3: smbd: Ensure get_real_filename() copes with empty pathnames.

Needed for vfs_glusterfs, as Gluster requires "." not '\0'.

Based on a fix from Anoop C S <anoopcs@redhat.com>

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13585

Signed-off-by: Jeremy Allison <jra@samba.org>
Reviewed-by: Ira Cooper <ira@samba.org>
Autobuild-User(master): Jeremy Allison <jra@samba.org>
Autobuild-Date(master): Wed Aug 22 21:50:41 CEST 2018 on sn-devel-144

(cherry picked from commit 9c71f61ed8a31d287d343d4f2e68cb40c57a2b89)

Autobuild-User(v4-7-test): Karolin Seeger <kseeger@samba.org>
Autobuild-Date(v4-7-test): Wed Aug 29 14:00:12 CEST 2018 on sn-devel-144