Add filter for cell layoutAttributes

In -[prepareForCollectionViewUpdates: prepareForCollectionViewUpdates:] the layoutAttributes for the footer accessory view were collected into _finalAnimationLayoutAttributesDict. This results in an -[NSArray addObject: nil] in -[PSTCollectionView indexPathsForVisibleItems] for a cell with no layoutAttributes.

By filtering the layoutAttributes in -[PSTCollectionViewData validateLayoutInRect:] to just the cell attributes we get the expected behavior.

(This might not be the best place for this check...)