[C++][Parquet] Minor: avoid re-create a new Reader object in Decoder::SetData

[mapleFU]

Describe the enhancement requested

  void SetData(int num_values, const uint8_t* data, int len) override {
    // num_values is equal to page's num_values, including null values in this page
    this->num_values_ = num_values;
    decoder_ = std::make_shared<::arrow::bit_util::BitReader>(data, len);
  }

Should be

  void SetData(int num_values, const uint8_t* data, int len) override {
    // num_values is equal to page's num_values, including null values in this page
    this->num_values_ = num_values;
    if (decoder_ == nullptr) {
      decoder_ = std::make_shared<::arrow::bit_util::BitReader>(data, len);
    } else {
      decoder_->Reset(data, len);
    }

Component(s)

C++, Parquet