fix: correct error mismatch causing race in fast walks

[lebauce]

Description

This PR is a follow up of #5482 and should have been part of it.
When using multiple workers, the same error is used by each of them, causing a data race.
err was used and is overwritten by the return value.

Related issues

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Checklist

• I've read the guidelines for contributing to this repository.
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• I've added tests that prove my fix is effective or that my feature works.
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[knqyf263]

Thanks