Non-mapreduce Riak search

> I'm not 100% sure whether this issue can occur in practice (the Riak docs were a bit sparse).

It can't, because we use `len(keys)` as the index into the results. The only way (short of a bug in Riak) to get `new_keys == []` is to have `len(keys) >= num_found`

I'll make the change you suggested, though.