readgroup: Don't use results if error occurs

If pr_ListMembers returns an error, don't try and use the results
it provides, as they could be garbage.

Caught by coverity (#92119)

Change-Id: Idcfb464953dcdec68cba988d3fb1ab6ddad08340
Reviewed-on: http://gerrit.openafs.org/9308
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>