message: Skip non-existent fetch nodes
diff: @@ -914,8 +914,8 @@ Status ConstantFolding::RunOptimizationPass(Cluster* cluster,
  // new names, and as a result users would not be able to fetch the node any
  // more with the original node name.
  for (const auto& fetch : item.fetch) {
  - auto fetch_node = node_map_->GetNode(fetch);
  - if (NumOutputs(*fetch_node) == 1) {
  + const NodeDef* fetch_node = node_map_->GetNode(fetch);
  + if (fetch_node && NumOutputs(*fetch_node) == 1) {
  nodes_whitelist_.insert(fetch_node->name());
  }
  }
  