Fix multiple cases of use of uninitialized vars

* Zeroing xevent helped address some erratic behavior.
* valgrind complained about using xfBitmap uninitialized
  during shutdown, traced it back to the initialization.
  Bitmap_Prototype->size > sizeof(rdpBitmap).
* Early exit from recv_tpkt_pdu is necessary to address
  a shutdown crash - the channelId value was being used
  without being set in the disconnect case.