ptserver: Don't print undefined value

ptserver: Don't print undefined value

The readgroup utility printed the wrong variable when listing the
source group - instead of printing the group name, it tried to
print an arbitrary element from the list of group members. Fix it
to print the right thing.

Caught by clang-analyzer.

Change-Id: Ie7ada8a1e881871ae5db14714ee10325ac052d40
Reviewed-on: http://gerrit.openafs.org/5076
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Tested-by: Jeffrey Altman <jaltman@openafs.org>