Don't assert indexForCell: API when cell isn't visible
Don't assert indexForCell: API when cell isn't visible

Summary:
Noticed this randomly debugging. Say you have a completion block that finishes after some async task happens. If you scroll the cell off screen it will return `nil` for `-[UICollectionView indexPathForCell:]`, and the assert will fail (unless the SC is at 0). Quiet some asserts.
Closes https://github.com/Instagram/IGListKit/pull/824

Differential Revision: D5303432

Pulled By: rnystrom

fbshipit-source-id: 64432c43f46d33ce052e6f4fedc094c1fc1f04e1

zhubofei/IGListKit,Instagram/IGListKit,Instagram/IGListKit,zhubofei/IGListKit,Instagram/IGListKit,Instagram/IGListKit,zhubofei/IGListKit,Instagram/IGListKit,zhubofei/IGListKit,zhubofei/IGListKit