restore: fix sys_wait4 error handling in case no child

Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

criu/pie/restorer.c

@@ -1024,7 +1024,7 @@ static int wait_helpers(struct task_restore_args *task_args)
 		pid_t pid = task_args->helpers[i];
 
 		/* Check that a helper completed. */
-		if (sys_wait4(pid, &status, 0, NULL) == -1) {
+		if (sys_wait4(pid, &status, 0, NULL) == -ECHILD) {
 			/* It has been waited in sigchld_handler */
 			continue;
 		}