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)

Change-Id: If70102e3da07135a9ec695f13caebe6298eff8ca
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>