fix: check end_idx before using it

[andylolz]

Fixes #125.

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Pull request checklist

• [x] I have linked my PR to an issue
• [x] I’ve used conventional commits
• [x] My branch is up-to-date with main
• [x] Where appropriate, I have added or updated tests
• [ ] ~Where appropriate, I have updated documentation to reflect my changes~

[andylolz]

The current CI issues should be resolved once #134 is in.

[andylolz]

This has ended up looking quite complicated, but it’s essentially a one line change: ba2bfb3eac7724cefd8809fefec5f6815d02c17c.

The rest is refactoring, tests and linting.