When terms don't have weight - don't need to sort them. Display the tree as is.

#3263 
The problem was that the terms don't have weight. That cause for the usort callback function to mess up the order of the terms.

I enter a fix that check first that the terms have a weight, if they have a weight we can use the sort the terms by their weight. 
![selection_024](https://f.cloud.github.com/assets/1222368/1066134/04d48b06-13ac-11e3-9c3c-236c6019bc51.png)