message: Avoid dereferencing a null pointer when the operation is missing.
diff: @@ -3606,16 +3606,18 @@ PyObject* TFE_Py_FastPathExecute_C(PyObject* args) {
  }
  TFE_Op* op = GetOp(ctx, op_name, op_exec_info.device_name, status);
  - tensorflow::unwrap(op)->SetStackTrace(tensorflow::GetStackTrace());
  auto cleaner = tensorflow::gtl::MakeCleanup([status, ctx, op] {
  ReturnStatus(status);
  ReturnOp(ctx, op);
  });
  +
  if (MaybeRaiseExceptionFromTFStatus(status, nullptr)) {
  return nullptr;
  }
  + tensorflow::unwrap(op)->SetStackTrace(tensorflow::GetStackTrace());
  +
  const tensorflow::OpDef* op_def = tensorflow::unwrap(op)->OpDef();
  if (op_def == nullptr) return nullptr;
  