message: fix bug in UndoMove
diff: @@ -105,7 +105,11 @@ bool MctsPlayer::UndoMove() {
  game_->UndoMove();
  if (!options_.tree_reuse) {
  // Reset the root to a fresh node.
  + if (root_->parent != nullptr) {
  *root_ = MctsNode(root_->parent, root_->move);
  + } else {
  + *root_ = MctsNode(root_->stats, root_->position);
  + }
  }
  return true;
  }
  