app: Fix bug when a tag that has no playlist is encountered

- Increase the formatting of playlist entries to allow up to 100000
  tracks. Also enforce that playlist entries are indexed by integers
  using the validate method.

- Add a validate method to validate the data stored in the
  btreedb. Optionally dump the contents to stdout. For testing, add a
  validate+dump by default when opening the db. This can be removed once
  the playlistdb is validated.

software/src/utils/playlistdb.py

@@ -192,7 +192,7 @@ class BTreeDB(IPlaylistDB):
                     print(f'Malformed playlist position: {val!r}')
                     result = False
                     continue
-                if 0 < idx or idx > last_pos:
+                if 0 > idx or idx > last_pos:
                     print(f'Playlist position out of range for {last_tag}: {idx}')
                     result = False
                 if dump: