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>

cr-service.c

@@ -212,7 +212,8 @@ static int restore_using_req(int sk, CriuOpts *req)
 
 	success = true;
 exit:
-	if (send_criu_restore_resp(sk, success, root_item->pid.real) == -1) {
+	if (send_criu_restore_resp(sk, success,
+				   root_item ? root_item->pid.real : -1) == -1) {
 		pr_perror("Can't send response");
 		success = false;
 	}