fs: fix negative offset when a suffix Range request exceeds object size

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A Range header requesting a suffix longer than the object (e.g.
"bytes=-90407" against a 5 byte object) caused RangeOption.Decode to
compute a negative offset (size - End), which serve.Object then used
directly as a slice/seek offset and panicked with "slice bounds out of
range". FixRangeOption (used by backends like OneDrive/Box that lack
native suffix-range support) had the same root cause: it produced a
RangeOption with a negative Start, which Header() silently dropped,
turning the request into the wrong byte range instead of erroring or
serving the whole object.

Per RFC 7233 section 2.1, when the suffix-length exceeds the
representation size, the entire representation should be served.
Clamp the computed offset/start to 0 in both places.

Fixes #6310

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500 Internal Server Error

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