pt_util: Protect against corrupt input files

If we have an input file which contains a group membership line
(with a leading space) before any group definitions occur, pt_util
would use stack garbage as the group to create these members in.

Avoid this by requiring the presence of a group line before any
membership lines.

Caught by coverity (#92180, #986000, #92248)

Change-Id: I536f089d67041dbf90f35b6dfeed5ba3303318a8
Reviewed-on: http://gerrit.openafs.org/9296
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>