libadmin: Don't pass garbage to pts_GroupCreate

The libadmin pts_GroupCreate function uses the value passed to
it in newGroupId to control whether the group is being created
with a user supplied ID or not.

Initialise this value in the caller, so we don't end up creating
groups with corrupt ids.

Caught by clang (#985978)

Change-Id: I5c3e2cec5b39b8eecbea316e351480863d3d7761
Reviewed-on: http://gerrit.openafs.org/9400
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>