`--frange=auto` exits with error message `Invalid 6581 filter range: -1` on certain songs

A few examples:
```
C64Music/MUSICIANS/C/Cadaver/Metal_Warrior_4.sid
C64Music/MUSICIANS/F/FAME/Chambers_of_Shaolin.sid
C64Music/MUSICIANS/H/Hogg_Ashley/CJs_Elephant_Antics.sid
C64Music/MUSICIANS/C/Cadaver/Abduction.sid
C64Music/MUSICIANS/W/Wizard_Oxygen/Fantasies.sid
C64Music/MUSICIANS/S/Siebold_Markus/Turrican_2-The_Final_Fight.sid
```