Assemble incoming data into per-line packets.

def _handle_lines(self):
        """Assemble incoming data into per-line packets."""
        while b'\xdd' in self._buffer:
            linebuf, self._buffer = self._buffer.rsplit(b'\xdd', 1)
            line = linebuf[-19:]
            self._buffer += linebuf[:-19]
            if self._valid_packet(line):
                self._handle_raw_packet(line)
            else:
                self.logger.warning('dropping invalid data: %s',
                                    binascii.hexlify(line))