pt_util: fix group line check for input files

Fix the check for requiring group lines before any membership lines. Do
not clear flag indicating the presence of a group after reading each
line.  (This error was caught by the pt_util-t unit test.)

Fix for commit 12ced70c95fe8efbcec09a372f0af81d819bb8cd

Reviewed-on: http://gerrit.openafs.org/9832
Reviewed-by: Simon Wilkinson <simonxwilkinson@gmail.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
(cherry picked from commit 9c391e7f7d0e5dfdfe947d385e2db9a8b8ca3610)

Change-Id: I6414f78d7611c61628aaacb4ae41111dbeec0d89
Reviewed-on: http://gerrit.openafs.org/11148
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>