Update BSCollectionController+BSItemsModel.m

When switching to an empty album and then back to one with content I ran into an issue with `scrollToItemAtIndexPath:`: 

`Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'attempt to scroll to invalid index path: <NSIndexPath: 0x19f5ad00> {length = 2, path = 0 - 0}'`

This fixed this problem without any side effects that I could spot