[IronPython] Bugfix for 2.7B2 allows pdb to run.

[Andrés Martinelli]

Hi,

  I have fixed a bug in 2.7B2 which prevented pdb from running.

  The problem is due to an exception thrown when accessing the
f_globals property of a frame.

  I've reported the bug details (
http://ironpython.codeplex.com/workitem/30177 ) and attached a patch
with a one line fix.

  Please review and let me know if I can provide any further assistance.

cheers,
Andrés