[Vanilla Bug] Fix crash when there's a wrong tiberium patch on a slope

[Rampastring]

Description:

Tiberian Sun crashes when trying to draw certain frames of tiberium overlay, meant for regular flat terrain, on slopes.

Steps To Reproduce:

1. Create a hill with slopes in FinalSun
2. Place tiberium on the slopes
3. Go in-game and reveal the area

Expected Behaviour:

The Tiberium should be drawn.

Actual Behaviour:

The game crashes.

Additional Files:

CnCNet ts-patches works around this crash by skipping drawing the incorrect frames: https://github.com/CnCNet/ts-patches/blob/master/src/tiberium_on_slope_crash.asm

It's not perfect, but better than having the game crash.