fix Crash detail:

[dishcool]

Fix: when CollectionView goto Reuse pool，need to set its provider = nil，otherwise there may be two collectionView have the same provider，and maybe will crash later. Fix：当CollectionView进入复用池，需要重置provider，避免出现 多个collectionView 持有同个provider对象，导致数据更新不同步，进而未同步的collectionView中flattenProvider的childSections范围错误导致越界崩溃