pstee: fix memory corrupation

The pstree_item for helpers is allocated without rst_info.
Before this patch prepare_fd_pid was executed for such items and
touched rst_info.
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

cr-restore.c

@@ -115,6 +115,9 @@ static int prepare_shared(void)
 		return -1;
 
 	for_each_pstree_item(pi) {
+		if (pi->state == TASK_HELPER)
+			continue;
+
 		ret = prepare_shmem_pid(pi->pid.virt);
 		if (ret < 0)
 			break;