Fix zero division ClassBar

[f1sch]

I was fighting Elisandre in Nighthold in Legion Remix with my Death Knight when I got the following error:

Message: ...ace/AddOns/Cell_UnitFrames/Widgets/Bars/ClassBar.lua:455: bad argument #1 to 'SetValue' (must be a finite number - Usage: self:SetValue(value))
Time: Tue Jan 13 20:15:22 2026
Count: 8
Stack:
[Interface/AddOns/Cell_UnitFrames/Widgets/Bars/ClassBar.lua]:455: in function 'UpdateRune'
[Interface/AddOns/Cell_UnitFrames/Widgets/Bars/ClassBar.lua]:471: in function 'callback'
[Interface/AddOns/Cell_UnitFrames/UnitFrames/EventMixin.lua]:52: in function '_OnEvent'
[Interface/AddOns/Cell_UnitFrames/UnitFrames/OnLoad.lua]:123: in function <...terface/AddOns/Cell_UnitFrames/UnitFrames/OnLoad.lua:110>

Locals:
bar = CUF_Player_ClassBarBar2 {
 index = 2
 PixelSnapDisabled = true
 border = Frame {
 }
 bg = Texture {
 }
}
start = 43861.497000
duration = 0
runeReady = false
timeLeft = -298.598000
progress = Infinite

I think Elisandes time rewind mechanics invalidates cooldown data or something like that.

The PR adds a Guard for division by zero and clamps timeLeft.
Progress is also checked for NaN and (negative) Infinity.