fix: out of bound panic in case of long deletions

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.02 :warning:

Comparison is base (a5c4a68) 88.87% compared to head (f4e2d5b) 88.85%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #91 +/- ## ========================================== - Coverage 88.87% 88.85% -0.02% ========================================== Files 19 19 Lines 7975 7980 +5 ========================================== + Hits 7088 7091 +3 - Misses 887 889 +2 ``` | [Impacted Files](https://codecov.io/gh/bihealth/hgvs-rs/pull/91?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=bihealth) | Coverage Δ | | |---|---|---| | [src/mapper/altseq.rs](https://codecov.io/gh/bihealth/hgvs-rs/pull/91?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=bihealth#diff-c3JjL21hcHBlci9hbHRzZXEucnM=) | `94.70% <100.00%> (-0.39%)` | :arrow_down: | 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=bihealth). 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=bihealth)

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