__len__ may not return a value less than zero

I've tried to find a relevant ticket a couple times now, but have been unable so I'm hoping I'm not being a fuss. I've got a relatively large installation (xapian-backed) and have noticed the occasional entry in the error tracker coming from haystack/query.py raising:

```python
ValueError: __len__() should return >= 0
````

Although I'm unsure why I'm encountering a SearchQuerySet with more ignored results than not, I figured it was a infrequently -encountered battle with invariants and worth reporting even if it were obvious.

Let me know if I've missed something, here.