[Cell/B.E.] Make parse_spufs_run more robust

[Cell/B.E.] Make parse_spufs_run more robust

With recent changes to inferior handling, parse_spufs_run needs to be
more careful in assumptions it makes.  In particular, this patch:

- Bails out early if the current inferior has not yet been registered
  (e.g. during fork procession) to avoid assertion failures in register
  cache code.

- Sets inferior_ptid to the current ptid while calling target_read_memory
  to make sure the correct process is accessed if parse_spufs_run is
  called early when inferior_ptid has not yet been switched by the caller.

ChangeLog:

	* spu-multiarch.c (parse_spufs_run): Bail out if inferior is not
	registered yet.  Set inferior_ptid while calling target_read_memory.