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)

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>
(cherry picked from commit 12ced70c95fe8efbcec09a372f0af81d819bb8cd)

Change-Id: I1ad524ed701e78ebe5b2e990168782a75d69613b
Reviewed-on: http://gerrit.openafs.org/11023
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>