reader: return Rock Ridge filename if available

[codecov[bot]]

Codecov Report

Patch coverage: 82.50% and project coverage change: +1.16 :tada:

Comparison is base (140311c) 77.47% compared to head (399e208) 78.63%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #34 +/- ## ========================================== + Coverage 77.47% 78.63% +1.16% ========================================== Files 5 6 +1 Lines 1123 1161 +38 ========================================== + Hits 870 913 +43 + Misses 178 175 -3 + Partials 75 73 -2 ``` | [Impacted Files](https://app.codecov.io/gh/kdomanski/iso9660/pull/34?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Kamil+Doma%C5%84ski) | Coverage Δ | | |---|---|---| | [image\_reader.go](https://app.codecov.io/gh/kdomanski/iso9660/pull/34?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Kamil+Doma%C5%84ski#diff-aW1hZ2VfcmVhZGVyLmdv) | `76.31% <75.00%> (-0.28%)` | :arrow_down: | | [rockridge.go](https://app.codecov.io/gh/kdomanski/iso9660/pull/34?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Kamil+Doma%C5%84ski#diff-cm9ja3JpZGdlLmdv) | `84.61% <84.61%> (ø)` | | | [susp.go](https://app.codecov.io/gh/kdomanski/iso9660/pull/34?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Kamil+Doma%C5%84ski#diff-c3VzcC5nbw==) | `100.00% <100.00%> (+10.08%)` | :arrow_up: |

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