Crash in prefetcher

[paulfreeman]

Describe the bug
When scrolling the PhotoView in the demo app the app crashed in the prefetcher call to the onCell prefetch method.

Crash occurred on onCellEvent?(.prefetchForData(data: dataToPrefetch)) in ASSectionDataSource

The call is on the background thread

	func prefetch(_ indexPaths: [IndexPath])
	{
		let dataToPrefetch: [Data] = indexPaths.compactMap
		{
			data[safe: $0.item]
		}
       onCellEvent?(.prefetchForData(data: dataToPrefetch)) 
	}

To Reproduce
Run the demo app, rapidly scroll through photo content until the crash occurs. (I had been testing the memory by going in and out of the AppStore Layout view repeatedly prior to this).

ASCollectionView version
Latest master (1.7)

Xcode Version:
12.1.1

Simulator, Device, Both?
iPhone 12 pro Max, iOS 14.1