F9.1 crashes in some cases

Created by: jkriege2

• F9.1 crashed with an exception, when a tag was a number only, as it is automatically converted to an int by the file-reader. This fix converts anything to a string first and checks the result then.
• this was found in this PR: https://github.com/KiCad/kicad-footprints/pull/236#issuecomment-356581010 & https://travis-ci.org/KiCad/kicad-footprints/builds/326992359?utm_source=github_status&utm_medium=notification