restore: check a return code of RUN_CLONE_RESTORE_FN

Otherwise we can skip an error and then criu will hang on switching to
the next stage.
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

criu/pie/restorer.c

@@ -1554,6 +1554,10 @@ long __export_restore_task(struct task_restore_args *args)
 			 */
 
 			RUN_CLONE_RESTORE_FN(ret, clone_flags, new_sp, parent_tid, thread_args, args->clone_restore_fn);
+			if (ret != thread_args[i].pid - 1) {
+				pr_err("Unable to create a thread: %ld", ret);
+				goto core_restore_end;
+			}
 		}
 
 		ret = sys_flock(fd, LOCK_UN);