Fixed bug in mmap of empty FixedSizeBinaryArray

[andy-thomason]

This PR fixes #1453

When mmap'ing a FixedSizeBinaryArray, the length in bytes was miscalculated.

This was only used for a check, but to avoid UB, this should be rigourous.

The problem manifested as an OutOfSpec error mapping zero length FixedSizeBinaryArray

This fix uses the DataType to determine the row length.

[jorgecarleitao]

Thank you @andy-thomason for this PR and well spotted! 🙇

[codecov[bot]]

Codecov Report

Patch coverage: 85.71% and project coverage change: -0.02 :warning:

Comparison is base (73ed7c8) 83.45% compared to head (5e2c52b) 83.43%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1454 +/- ## ========================================== - Coverage 83.45% 83.43% -0.02% ========================================== Files 376 376 Lines 41161 41166 +5 ========================================== - Hits 34350 34346 -4 - Misses 6811 6820 +9 ``` | [Impacted Files](https://codecov.io/gh/jorgecarleitao/arrow2/pull/1454?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jorge+Leitao) | Coverage Δ | | |---|---|---| | [src/io/ipc/read/error.rs](https://codecov.io/gh/jorgecarleitao/arrow2/pull/1454?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jorge+Leitao#diff-c3JjL2lvL2lwYy9yZWFkL2Vycm9yLnJz) | `100.00% <ø> (ø)` | | | [src/mmap/array.rs](https://codecov.io/gh/jorgecarleitao/arrow2/pull/1454?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jorge+Leitao#diff-c3JjL21tYXAvYXJyYXkucnM=) | `96.52% <85.71%> (-0.19%)` | :arrow_down: | ... and [4 files with indirect coverage changes](https://codecov.io/gh/jorgecarleitao/arrow2/pull/1454/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jorge+Leitao) Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jorge+Leitao). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jorge+Leitao)

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