Fix #26021 - Issues with non-conformant Figured Bass entries

Figured Bass which do not conform to F.B. 'rules' have several issues. See http://musescore.org/en/node/26021 for details.
Merge pull request #968 from mgavioli/Fix_26021_Issues_with_non-conformant_FB_entries

Fix #26021 - Issues with non-conformant Figured Bass entries