use checked addition to not panic in debug build

[Skgland]

used checked addition so this does not panic in debug builds

related to issue #870

not sure if #870 is completely resolved as there are other comments indicating a possible overflow condition that I did not understand and as such did not fix

[Skgland]

Its also probably not ideal tha the fixed code and the test are in different crates, but I didn't know how to separate the punycode from the url and how to add a test in the punycode crate as that appears to use a custom testing framework

[Skgland]

not sure if #870 is completely resolved as there are other comments indicating a possible overflow condition that I did not understand and as such did not fix

I think the line I was refering to is fine: min_code_point - code_point > (u32::MAX - delta) / (processed + 1)

min_code_point should be lager than code_point due to the iterator filter c >= code_point as such this can't underflow on min_code_point - code_point

u32::MAX - delta cannot underflow either as delta can't be larger than u32:MAX

process + 1 can't overflow as process < input_length per the while condition

process + 1 is not 0 as process is unsigned, so no division by zero

[Skgland]

Restructured changes and moved test to the idna creat where it belongs in my opinion.

• The first commit fixes the line endings of debug_metadata/README.md as it was always considered dirty and prevented me from rebasing otherwise.
• The second commit adds the test, fails without the third commit.
• The third and last commit fixed the panic in debug builds due to overflow.

[Skgland]

Rebase again as I accidentally hardcoded the string in the Bad Punycode test instead of using the value(s) from bad_punycode_tests.json.

[Skgland]

This should now be ready for review again.

[codecov[bot]]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (e39c9a2) 81.81% compared to head (336623e) 81.84%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #877 +/- ## ========================================== + Coverage 81.81% 81.84% +0.03% ========================================== Files 20 20 Lines 3514 3531 +17 ========================================== + Hits 2875 2890 +15 - Misses 639 641 +2 ``` | [Files](https://app.codecov.io/gh/servo/rust-url/pull/877?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=servo) | Coverage Δ | | |---|---|---| | [idna/src/punycode.rs](https://app.codecov.io/gh/servo/rust-url/pull/877?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=servo#diff-aWRuYS9zcmMvcHVueWNvZGUucnM=) | `85.44% <100.00%> (+0.44%)` | :arrow_up: | | [idna/tests/punycode.rs](https://app.codecov.io/gh/servo/rust-url/pull/877?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=servo#diff-aWRuYS90ZXN0cy9wdW55Y29kZS5ycw==) | `80.39% <78.94%> (-0.86%)` | :arrow_down: | ... and [1 file with indirect coverage changes](https://app.codecov.io/gh/servo/rust-url/pull/877/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=servo)

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[Skgland]

Fixed added test to not increase msrv beyond 1.56 (I think this change also increases readability of the test) and fixed unrelated clippy lint. Ready for review again.

[Skgland]

Now with fixed formatting. Ready for review again, sorry for the noise.

[Skgland]

I messed up my rebase, one moment.

[Skgland]

Ok, now the rebase should be fixed.