[Cython] crash bug for closures on extension type methods

Hi,

yes, I know - right after a release is the best time to find crash bugs. 
Here is one that I find particularly unpleasant:

http://trac.cython.org/cython_trac/ticket/742

Given that this was broken for basically forever without anyone noticing, 
this won't qualify for an emergency release, but I think we shouldn't wait 
too long with a follow-up release to get this out.

Stefan