default for missing post_index parameter

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)
Merge pull request #2119 from Fusselwurm/patch-1

default for missing post_index parameter when topic pagination is used
Merge branch '0.5.1' into hashtalk

Conflicts:
	src/controllers/topics.js