Fix missing font index with some XLS files

[jenasnic]

This is:

• a bugfix (I got an error when trying to load an XLS file from an Italian agency : missing fonts? File can be easily opened in Microsoft Office or Open Office)
• a new feature
• refactoring
• additional unit tests

Checklist:

• Changes are covered by unit tests
  • Changes are covered by existing unit tests
  • New unit tests have been added
• Code style is respected
• Commit message explains why the change is made (see https://github.com/erlang/otp/wiki/Writing-good-commit-messages)
• CHANGELOG.md contains a short summary of the change and a link to the pull request if applicable
• Documentation is updated as necessary

Why this change is needed?

Provide an explanation of why this change is needed, with links to any Issues (if appropriate).
If this is a bugfix or a new feature, and there are no existing Issues, then please also create an issue that will make it easier to track progress with this PR.

[oleibman]

Is it possible for you to create a test case that works with your updated code but fails with the code you're replacing?

[jenasnic]

Hello.

Thanks for your quick response!
Here is a file to reproduce bug.
For GDPR reason, file has been anonymized using VIM (when editing/saving file with Microsoft Office or Open Office or LibreOffice, error disappears).
The original file comes from a customer in Italy and it has probably been created using Microsoft Excel (we ignore which version).

The error occurs when loading Spreadsheet (missing loaded fonts in PhpOffice\PhpSpreadsheet\Reader\Xls.php?) :

    $file = new \SplFileInfo('test_ko.xls');
    IOFactory::load($file);

I create TestCase as asking.

Best regards!

test_ko.xls

[oleibman]

Thank you for your contribution.