pts: Don't assume that pr_IdToName() was successful

If pr_IdToName fails for some reason, tnames won't be available.

Change-Id: I5270ba0ecf3cfad9bfb909c97b27473d96ca1307
Reviewed-on: http://gerrit.openafs.org/10952
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: D Brashear <shadow@your-file-system.com>