code: function BufferSegment(buf, offset, count) {
    if (!Buffer.isBuffer(buf)) throw new TypeError('buf must be a buffer');
  
    this.buffer = buf;
    this.offset = offset || 0;
    this.count = count || buf.length;
  }
docs: Create a buffer segment
  @private
  @param {Buffer} buf
  @param {number} [offset]
  @param {number} [count]
  @constructor