fix panic on `xn--55555577`

[Byron]

This PR fixes a panic discovered by clusterfuzz via gitoxide (PR).

Details can be found in https://github.com/Byron/gitoxide/issues/1401 (but only once the bug is considered fixed by the fuzzer).

Notes for the Reviewer

• The fix assumes that this is indeed an invalid punycode sequence, which is why it's made to error.
• The panic would only happen in debug mode, or if compiled with integer overflow checks specifically. However, this may indeed be the case for some applications which require this extra-levle of safety.
• A new patch release would be appreciated - the fuzzer will keep sending me emails otherwise as it seems to 'bounce' from fixed to not-fixed on this one.

[codecov[bot]]

Codecov Report

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #940 +/- ## ======================================= Coverage ? 79.83% ======================================= Files ? 23 Lines ? 4220 Branches ? 0 ======================================= Hits ? 3369 Misses ? 851 Partials ? 0 ```

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