ptuser: Clear return value from pr_ListMembers

Make sure that lnames is zero'd on entry to pr_ListMembers, so that
an early error return doesn't also return garbage.

Reviewed-on: http://gerrit.openafs.org/9309
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
(cherry picked from commit bc70828f0713891597cf0042bc3e204f3d923271)

Change-Id: If99abc35f49ac1d436fbb82c1f9dbf97590e95c5
Reviewed-on: http://gerrit.openafs.org/9379
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Paul Smeddle <paul.smeddle@gmail.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>