[3.0] xiph_metadata: ensure the tracks metadata are valid numbers
They are already parsed as unsigned values in the TRACKNUMBER field. The UTF-8 check was removed from these fields in dfdc98f7. If we turn them into a valid integer (when they are) we have safe UTF-8 data.
Ref. #28976
Backport of !7595 (merged)
(cherry picked from commit 63c36ef5)