Method underflow

cpp/fory/util/buffer_test.cc:52–59

  }

  int_type underflow() override {
    if (pos_ >= data_.size()) {
      return traits_type::eof();
    }
    current_ = static_cast<char>(data_[pos_]);
    setg(&current_, &current_, &current_ + 1);
    return traits_type::to_int_type(current_);
  }

private:
  std::vector<uint8_t> data_;