Flush outgoing data which OpenSSL put in our BIO to the transport.

async def _write_ssl(self):
        """
        Flush outgoing data which OpenSSL put in our BIO to the transport.
        """
        pending = lib.BIO_ctrl_pending(self.write_bio)
        if pending > 0:
            result = lib.BIO_read(self.write_bio, self.write_cdata, len(self.write_cdata))
            await self.transport._send(ffi.buffer(self.write_cdata)[0:result])
            self.__tx_bytes += result
            self.__tx_packets += 1