Summary:
Fix https://github.com/facebook/rocksdb/issues/6470

Ensure TransactionLogIter being initialized correctly with SYNC_POINT API when calling `GetSortedWALFiles`.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/11725

Reviewed By: akankshamahajan15

Differential Revision: D48529411

Pulled By: ajkr

fbshipit-source-id: 970ca1a6259ed996c6d87f7fcd40f95acf441517