getPayloadLength64(cb) {
    if (this._bufferedBytes < 8) {
      this._loop = false;
      return;
    }

    const buf = this.consume(8);
    const num = buf.readUInt32BE(0);

    //
    // The maximum safe integer in JavaScript is 2^53 - 1. An error is returned
    // if payload length is greater than this number.
    //
    if (num > Math.pow(2, 53 - 32) - 1) {
      const error = this.createError(
        RangeError,
        'Unsupported WebSocket frame: payload length > 2^53 - 1',
        false,
        1009,
        'WS_ERR_UNSUPPORTED_DATA_PAYLOAD_LENGTH'
      );

      cb(error);
      return;
    }

    this._payloadLength = num * Math.pow(2, 32) + buf.readUInt32BE(4);
    this.haveLength(cb);
  }
Gets extended payload length (7+64).

@param {Function} cb Callback
@private