message: Fix possible segfault in shape function for SymbolicGradient
diff: @@ -40,7 +40,11 @@ REGISTER_OP("SymbolicGradient")
  if (types[i] == DT_RESOURCE) {
  const std::vector<shape_inference::ShapeAndType>* handle_type =
  c->input_handle_shapes_and_types(i);
  + if (handle_type != nullptr) {
  c->set_output(i, handle_type->at(0).shape);
  + } else {
  + c->set_output(i, c->UnknownShape());
  + }
  } else {
  c->set_output(i, c->input(i));
  }
  