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

Change-Id: Iaa47e3ab7936668e8a5b761dd8d3f933d459a16a
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>