remove method on IntIntMultiMap fixed

While I was testing HugeHashMap, I noticed that the remove method on the HugeHashMap does not work in some specific cases. After some investigation I found an issue in IntIntMultiMap causing the issue in HugeHashMap. This pull request provides a possible fix for this issue. Please review the fix or provide an alternative.

I'd like to make more changes to HugeHashMap - I'll issue another pull request once it is decided about this request.

Regards,
Jaromir