[storm] storm cextensions fixes

[Gustavo Niemeyer]

> I checked out your latest need-for-speed-revenge branch, and had two
> immediate problems:
>  * the test suite segfaulted in Compile__call__()
>  * the extension would not compile with Python 2.4
> 
> Attached is a bundle that fixes both issues.

Oops.. it wasn't segfaulting for me unfortunately.  I guess that proves
the point about single-exit functions.

I'm sorry about that, and thanks for the fixes.

-- 
Gustavo Niemeyer
http://niemeyer.net