Reset inner_cur of (multipass) flatten_adaptor when reaching the end

[tcbrindle]

Fixes #150

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (e942a67) 97.99% compared to head (1fc9de3) 97.99%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #151 +/- ## ======================================= Coverage 97.99% 97.99% ======================================= Files 66 66 Lines 2389 2392 +3 ======================================= + Hits 2341 2344 +3 Misses 48 48 ```

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