Incorrect array bounds check in NPOI.XSSF.Model.StylesTable.GetCellStyleXfAt

[aboryczko]

Hi,
I was trying to use the the Npoi.Mapper library on an XLSX file generated by LargeXlsx which generates a very basic style sheet file which doesn't contain any cellStyleXfs. The current code:

internal CT_Xf GetCellStyleXfAt(int idx)
{
    if (idx < 0 || idx > styleXfs.Count)
        return null;
    return styleXfs[idx];
}

fails when styleXfs is empty because it checks idx > styleXfs.Count instead of idx >= styleXfs.Count.
I will create a PR with the fix and test case.

Kind regards,
Artur