ZKAPs aren't spent on allocate_buckets if the server has no shares

#160 changed the client-side allocate_buckets ZKAP spending logic so that if a server already had some shares the client tried to upload, the client would not consider ZKAPs spent for those shares, only for those it really needs to upload.

However, the logic was incorrect and caused the client to consider no ZKAPs spent if the server had no shares already.