avoid crashing: when filter combine is already deinitialized, but still processing (weak==nil)

[elisar4]

I am getting crashed while doing simple steps:

1. I have 2 view controllers first is main, second is filter controller
2. present filter controller from main controller
3. previews of filters are starting processing
4. before processing complete click "close" button to dismiss filter controller
5. callback "processedPreviewImagesCompletion" or "processedImagesCompletion" will now crash the app, when processing finished, because filter controller is already deallocated

The same issue could appear in all other callbacks with weak. These two are the most commonly used and with this commit it will not crash the app anymore.