I think it shouldn't be possible to up or down vote a closed question.
Here is an example: Ask a stupid off topic question, and ask someone to quickly answer it before it is closed by moderator. You don't have the possibility to delete it (because it has an answer), and you will lose reputation for this until the end of time (each time someone will fall on it).
EDIT : OK, I have to explain it carefully : The example up here was "how to make it to test the situation I talked about". Ok Ok Ok. That was not clear for everyone.
Here is a better explanation :
You ask a question that you find really interesting, but you didn't notice that it was quite off topic on the SE website.
Someone answers your question, because he find also that this question is interesting or he/she had the same problem once.
Other people read your question and ask for closing
Your question is closed and commented as "off topic"
You agree upon this closing and since it's already -12, you want to stop the get bad reputation and decide to delete your question
The system tells you that you can NOT delete your question because there is a answer with upvotes.
As a consequence, you will lose rep until the end of time.
Is this more clear ?