fix: error where `aead::DecryptReader` always errors if input `Read` len is less than `Segment` size

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.17 :tada:

Comparison is base (763f349) 59.14% compared to head (f040556) 59.31%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #39 +/- ## ========================================== + Coverage 59.14% 59.31% +0.17% ========================================== Files 88 88 Lines 9164 9188 +24 ========================================== + Hits 5420 5450 +30 + Misses 3744 3738 -6 ``` | [Impacted Files](https://codecov.io/gh/chanced/navajo/pull/39?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Chance) | Coverage Δ | | |---|---|---| | [navajo/src/aead/reader.rs](https://codecov.io/gh/chanced/navajo/pull/39?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Chance#diff-bmF2YWpvL3NyYy9hZWFkL3JlYWRlci5ycw==) | `98.36% <100.00%> (+4.48%)` | :arrow_up: | ... and [1 file with indirect coverage changes](https://codecov.io/gh/chanced/navajo/pull/39/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Chance) 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=Chance). 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=Chance)

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