Kovri: potential buffer over-read in garlic clove handling + I2NP message creation

Brief ----- There is a lack of sanitation checks when handling Garlic messages in the kovri I2P router. Sending a specially crafted Garlic message can cause the router to send onward an I2P message containing leaked RAM data, triggering a massive information leakage.

    GarlicDeliveryType delivery_type = (GarlicDeliveryType)((flag >> 5) & 0x03);
    switch (delivery_type) {
      case eGarlicDeliveryTypeLocal:
        LOG(debug) << "GarlicDestination: Garlic type local";
        HandleI2NPMessage(buf, len, from);
      break;
      case …