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.

Change-Id: I78c35f0c18d9c7ec3f638c8e80609ac28d362925
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>