Fix software splats breaking and crashing in skyboxes

Fixes out-of-bounds memory reads in R_ClipVisSprite that cause glitches and crashes when rendering floor sprites in skyboxes or portals.

Method for reproducing the crash:

1. Load this script, which puts a blue sphere floor sprite into GFZ1's skybox. sw-splat-skybox-crash.lua

2. Load GFZ1.

3. Go to this spot, move and look around and it should crash pretty fast. (It does on my computer at least)

Test build: (based on next so be careful with your save files)

srb2win_sw-splat-skybox-crash.exe