fix: fix custom instrument data lookup using wrong index

Custom instruments start at `song.header.default_instruments` (currently 16), so when looking up custom instrument data, the default instrument count must be subtracted from the note's instrument ID.

Author: Bentroen

nbswave/main.py

@@ -170,7 +170,8 @@ def _mix(
                     sound1 = self._instruments[note.instrument]
                 except KeyError:  # Sound file missing
                     if not ignore_missing_instruments:
-                        instrument_data = self._song.instruments[ins]
+                        custom_ins_id = ins - self._song.header.default_instruments
+                        instrument_data = self._song.instruments[custom_ins_id]
                         ins_name = instrument_data.name
                         ins_file = instrument_data.file
                         raise MissingInstrumentException(