message: fix segfault
  Segfault was seen if NGRAPH_TF_USE_PREFETCH was set in a script that did not have a MakeIterator/PrefetchOp
diff: @@ -114,6 +114,13 @@ Status CaptureVariables(Graph* graph, std::set<string> skip_these_nodes) {
  // If Prefetch is requested
  if (std::getenv(NGraphPrefetchSharedResouce::NGRAPH_TF_USE_PREFETCH) !=
  nullptr) {
  + if (make_iterator_nodes.size() == 0) {
  + // No MakeIterator Op found in the Graph
  + make_iterator_nodes.clear();
  + nodes_to_capture.clear();
  + return Status::OK();
  + }
  +
  if (make_iterator_nodes.size() > 1) {
  return errors::Internal(
  "Found more than 1 MakeIterator nodes. This case is not supported.");
  