message: [segfault]: root_node nullptr check
diff: @@ -637,6 +637,7 @@ bool HloParserImpl::ParseInstructionList(HloComputation** computation,
  if (root_node == nullptr) {
  LOG(FATAL) << "instruction " << root_name
  << " was marked as ROOT but the parser has not seen it before";
  + return false;
  }
  root = root_node->first;
  }
  