East & south facing walls render incorrectly on block edges

[makeusabrew]

It seems that when a ray hits an east or south facing wall (e.g. the player is facing west or north) that the wrong distance is chosen on block edges (e.g. when the ray exactly hits the start/end of a block). Must be a rounding issue somewhere as doesn't happen when facing south or east e.g. when casting in a 'positive' direction.