Fixed items.previousValue undefined check in ngOnChanges
Fixed items.previousValue undefined check in ngOnChanges

The current ngOnChanges implementation does not check which `Input` property is being changed - it could be that the `childHeight` property is being changed.
As a result, I was getting an exception `Cannot read property 'length' of undefined` -  in this case `items.previousValue.length`.
I have added a check to prevent this exception.
rintoj/angular2-virtual-scroll