Looping parent / child categories breaks site

[mardala]

To reproduce:

1. Create a category.
2. Create another category as a child of the first category.
3. Make the first category a child of the 2nd category.

Granted you wouldn't normally do this, but if 'by accident' you did this, nothing stops you from doing this and a. you get a blank page where your site was and b. in the admin you cannot undue this. The categories disappear. So its a db fix from what I did. Granted I did this on my dev server just to see what would happen and not on a production server. But in case you accidentally did this to a live site you'd be scrambling to fix it.

[scribu]

This was fixed in trunk, I believe. See #14662