Make sure os is not even partly destroyed

[ddanier]

I had problems with errors like:
Exception TypeError: "'NoneType' object is not callable" in > ignored

After much digging I found, that os.kill is None in this case while os still exists. No clue how Python managed to get this state, but my change fixes the problem.

[Byron]

Thank you very much ! It's a shame python is so unreliable when the process is on it's way down, and even though I believe they have good reasons for that, just removing standard library functions is certainly a terrible way to communicate these.