Bound flatCols to MaxColumns when flattening worksheet columns

[arpitjain099]

flatCols materializes one xlsxCol per column in the range a <col> element declares. Those min and max attributes come straight from the worksheet XML and are not validated anywhere on the way in, so the size of that loop is whatever the file says it is.

A worksheet cannot hold more than MaxColumns columns, so flattening past that bound cannot produce anything usable. This clamps both loops to MinColumns..MaxColumns.

For any range a worksheet can genuinely contain the result is unchanged, since the clamp only takes effect outside the representable range. The four column mutators that call flatCols (setColVisible, setColOutlineLevel, setColStyle, setColWidth) keep their signatures, so nothing changes for callers.

I went with clamping rather than returning ErrColumnNumber to keep this to flatCols. Returning an error would mean threading it through those four helpers and their nine call sites across col.go and stream.go. Happy to redo it that way if you would rather reject such a file outright, and equally happy to move the check to parse time instead.

Verified:

• go build ./... clean, gofmt clean
• full go test ./... passes (71s)
• added TestFlatColsBounded, which sets a <col> range of 1..2147483647 on a sheet, calls SetColWidth, and asserts the flattened set stays within MaxColumns while a real column is still flattened and readable

Filed at your request from the report on the advisory.

[xuri]

Thanks for your contribution. I've made some changes based on your branch.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.62%. Comparing base (db93f8d) to head (bac5891).

Additional details and impacted files

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##           master    #2370   +/-   ##
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  Coverage   99.62%   99.62%           
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  Files          32       32           
  Lines       26994    26994           
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  Hits        26892    26892           
  Misses         53       53           
  Partials       49       49           

Flag	Coverage Δ
unittests	99.62% <100.00%> (ø)

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