Corrected length of the PARTY_BOOKING_REQ_REGISTER packet for Ragexe clients

- Fixes bugreport:7656 (thanks to Angelmelody)
  http://hercules.ws/board/tracker/issue-7656-party-booking-system-unsupported-packet-0x0000-0x1b01
- While I have not checked all of the affected clients, the ones I
  checked had a length of 18. The 41-byte version appears to be used by
  the RagexeRE clients only. If this speculation turns out to be
  incorrect, or your client is incompatible with this patch, please
  report it and it'll be fixed.

Signed-off-by: Haru <haru@dotalux.com>