Arsenal - Fix Bad Vehicle Type error for backpack

[m1kesk]

When merged this pull request will:

• Will check if backpack class exist before is set to setLoadout array, preventing Bad Vehicle Type error

[johnb432]

I'm a bit confused. When is this a problem? If there's a backpack that doesn't exist, then it shouldn't even appear in the arsenal - if it does, we might be able to address it at the root.

[PabstMirror]

the ace arsenal from main menu will try to load last loadout and throws that error often with changing modsets
https://github.com/acemod/ACE3/blob/master/addons/arsenal/missions/Arsenal.VR/initPlayerLocal.sqf#L6-L11

I think that might be the root problem, otherwise I'm not sure how bad backpacks are getting into the list data

[johnb432]

the ace arsenal from main menu will try to load last loadout and throws that error often with changing modsets https://github.com/acemod/ACE3/blob/master/addons/arsenal/missions/Arsenal.VR/initPlayerLocal.sqf#L6-L11

I think that might be the root problem, otherwise I'm not sure how bad backpacks are getting into the list data

If it's only that, we can easily address there.