Fix arithmetic exception when fading out plasma

[voidanix]

When applying plasmapartfade1 to 1, when the particles fade out, the game crashes with an arithmetic exception

Thanks to @robalni for unintentionally finding this

[MoonPadUSer]

does this happen with other weapons too? (would be a little weird otherwise, wouldn't it)

[robalni]

I just tested it and it seems this only happens for plasma.

[ghost]

Is there is a reason here to think "f == fmax();" everytime?

[voidanix]

Is there is a reason here to think "f == fmax();" everytime?

no idea, although when replacing rnd() and rndscale() with mt19937 i noticed that the particles never fade away (and the game doesn't crash)