[Stackless] slp_kill_tasks_with_stacks is broken

[Richard Tew]

The test_threading.py crash appears to have no frame, which causes the
previously mentioned infinite loop.  Here's my fix which I believe to
be correct for the current (and much maligned) approach:

  http://hg.python.org/stackless/rev/518bc9308f7b

2.7.1 appears to have another crash bug.  This is pretty much:

  * Run Stackless unit tests.
  * When interpreter exits, it crashes.
  * A list has one item which appears to be uninitialised.
  * The crash occurs when this list is garbage collected.

This should be something in the Stackless unit tests, so I am not too
concerned about holding up the 2.7.2 release to fix it.  I still have
to include the one outstanding bug fix to the stack chaining, and that
should be it.

Cheers,
Richard.