default for missing post_index parameter when topic pagination is used

Avoid getting NaN if post_index param is not set (NaN is passed through to redis, redis complains, and the user ends up at /404).

( I switched from 0.5.0-r4 to the 0.5.1 branch for the sake of having subcategories, and suddenly /topic/:tid/:slug gave me 404s  :D )