during unset(), the destructors for other objects with cyclic references can get triggered, resulting in the functions being reentered before the count() call. This leads to a crash because the offset no longer exists.
Instead, we check if only the given PermissibleInternal is present, and clean everything up with a single unset instead of two.
This could also have been solved by adding extra isset() checks before checking the counts, but this way seemed more elegant.
This is similar to an issue with AsyncTask thread-local storage a few months ago, which was also caused by GC reentrancy.
closes#6119
