Clean up return handling for a number of SPX-related routines that were derived from more complex TCP versions of the same:
Clean up return handling for a number of SPX-related routines that
were derived from more complex TCP versions of the same:

- spx_close(), spx_disconnect(), spx_drop(), and spx_usrclosed() all
  always free's the spxpcb invalidating the argument, so a return
  value is not required to indicate if it has.
- Annotate that the cb arguments to each of these functions is
  invalidated via a comment.
- When tearing down a pcb due to sonewconn() having failed, mark the
  cb as NULL; later, when deciding whether to store trace information
  due to SO_DEBUG, check that cb is not NULL before dereferencing or
  a NULL pointer dereference may occur.

MFC after:	3 weeks

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase