message: Prevent null pointer dereference in constant folding.
  Under certain conditions, an invalid protobuf saved model with invalid nodes would be loaded. During optimization phase, Grappler optimizer will then dereference a null pointer.
diff: @@ -3476,6 +3476,9 @@ bool ConstantFolding::MulConvPushDown(GraphDef* optimized_graph, NodeDef* node,
  NodeDef* mul_left_child = node_map_->GetNode(node->input(0));
  NodeDef* mul_right_child = node_map_->GetNode(node->input(1));
  + if (mul_left_child == nullptr || mul_right_child == nullptr) {
  + return false;
  + }
  // One child must be constant, and the second must be Conv op.
  const bool left_child_is_constant = IsReallyConstant(*mul_left_child);
  const bool right_child_is_constant = IsReallyConstant(*mul_right_child);
  