message: Reduce the risk of crash by checking hook interface in ApiCallback.
diff: @@ -1513,6 +1513,7 @@ Status CuptiTracer::HandleCallback(CUpti_CallbackDomain domain,
  CUpti_CallbackId cbid,
  const CUpti_CallbackData *cbdata) {
  if (!api_tracing_enabled_) return Status::OK(); // already unsubscribed.
  + if (!cupti_driver_api_hook_) return Status::OK(); // already unsubscribed.
  if (domain != CUPTI_CB_DOMAIN_DRIVER_API) return Status::OK();
  if (internalCuCall) return Status::OK();
  