It's cleaned up accoding with following statements:
* files_id can't be zero (look at dump_task_kobj_ids)
* item->ids is allocated for all non-dead tasks
* a parent can't be dead

In addition here is a tiny coding stype fix.

Fixes: 475bb1e7 ("rst: Evaluate per-task clone mask early")
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>