message: Fix accounting bug in closes.
  Summary:
  We didn't actually ensure there was enough room for the tag here, which means we'd fail to encrypt the close.
  (Note: this ignores all push blocking failures!)
diff: @@ -1176,6 +1176,8 @@ void writeCloseCommon(
  }
  auto packet = std::move(packetBuilder).buildPacket();
  packet.header->coalesce();
  + packet.body->reserve(0, aead.getCipherOverhead());
  + CHECK_GE(packet.body->tailroom(), aead.getCipherOverhead());
  auto body = aead.inplaceEncrypt(
  std::move(packet.body), packet.header.get(), packetNum);
  body->coalesce();
  