Addsheet May Leave Active Sheet Uninitialized

[oleibman]

Fix #4112. Direct cause is that applyStylesFromArray tries to save and restore activeSheetIndex. However, if activeSheetIndex is -1, indicating no active sheet, the restore should not be attempted. Code is changed to test before attempting to restore.

The actual problem, however, is that user specified a sheet number for addSheet. That method will set activeSheetIndex most of the time, but this was a gap - when the supplied sheet number (0 in this case) is greater than activeSheetIndex (-1 in this case), it was leaving activeSheetIndex as -1. It is changed to set activeSheetIndex to 0 when activeSheetIndex is negative.

This is:

• a bugfix
• 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.