virtualMass - Fix div0 error when naked

[PabstMirror]

Playing as animal or human and removing everything throws div0 error as load _unit is 0.

@commy2 as best I can tell loadAbs _unit / load _unit; is the same as the config

class CfgInventoryGlobalVariable {
    maxSoldierLoad = 1000;
};

So using that should be faster and prevent the error.