Bugfix
   * Fix the alert raised when a client requests an invalid
   * fragment length, as per RFC6066 section 4. We now alert with
   * MBEDTLS_SSL_ALERT_MSG_ILLEGAL_PARAMETER. Similarly, raise
   * MBEDTLS_SSL_ALERT_MSG_DECRYPT_ERROR for an invalid finished
   * message, as per RFC5247 section 7.2.2.