Fix issue from r166732 found by Andrew Kaylor

From Jim Ingham's email:
  It does look like ThreadPlanStepInRange test is some kind of thinko.
  In practice, in this case it is probably safe to run only one thread
  when doing the "step through" since that generally involved running
  from a shared library stub to its target.  That could deadlock if the
  dynamic loader has to fix up the symbol, and another thread is in the
  middle of doing that.  But that doesn't seem to be very common, or at
  least the code is clearly wrong but I haven't had any reports of this
  causing deadlocks...