[Bug] fix overflow while convert fixed char to number

[stdpain]

because Slice::size was unsigned int64, p >= 0 was always true

Proposed changes

Fix potential overflow issues

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Checklist

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• I have created an issue on (Fix #ISSUE) and described the bug/feature there in detail
• Compiling and unit tests pass locally with my changes
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Further comments

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[morningman]

LGTM

[github-actions]

PR approved by at least one committer and no changes requested.

[github-actions]

PR approved by anyone and no changes requested.