Sprites are sometimes visible through walls

[viciious]

https://github.com/team-eternity/calico-doom/blob/master/src/r_phase2.c#L155

The case when b_floorheight equals 0 isn't properly handled, so depending on the relative height of the floor, sprites would sometimes clip through walls. The first part of the condition should actually be b_floorheight >= 0 && b_floorheight > f_floorheight.