common: set errno=0 before calling strto{l,ul,ull}

[whitslack]

The strto{l,ul,ull} functions do not set errno upon a successful return, so a successful return from a maximally valued input could be misinterpreted as an overflow error if errno happened already to be set to ERANGE before the call. To guard against this edge case, always set errno to zero before calling these functions if checking errno afterward.

Checklist

Before submitting the PR, ensure the following tasks are completed. If an item is not applicable to your PR, please mark it as checked:

• The changelog has been updated in the relevant commit(s) according to the guidelines. Trivial fix warrants no changelog message.
• Tests have been added or modified to reflect the changes. Don't think it's worth writing tests for this edge case.
• Documentation has been reviewed and updated as needed. There's no documentation to update.
• Related issues have been listed and linked, including any that this PR closes. I doubt this has been reported, or else it would already have been fixed (I hope), but I didn't check.