libadmin: Don't free garbage pointer

If we jump to the error handler early on in pts_GroupOwnerChange,
idlist may not have been used, and so we will end up trying to
free stack garbage.

Initialise the structure to 0 at the start of the function, so it
is always safe to enter the error handler.

Caught by coverity (#985962)

Reviewed-on: http://gerrit.openafs.org/9398
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 8260d86dda766a21e9f457994e7a3b259ba3a31b)

Change-Id: I79f7fe3cba11a2904c644e6747511e3630d74a79
Reviewed-on: http://gerrit.openafs.org/11047
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>