service: fix bug, when root_item isn't initialized

If restore fails on early stage(like no images in directory), then root_item
might be uninitialized, so when we are trying to send response with root_item->pid
criu crashes.

Signed-off-by: Ruslan Kuprieiev <kupruser@gmail.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>