#56 some subsongs won't play

[Hugh Allen]

I tested with the following versions of Sidplay:

- Sidplay 1/w win32 GUI (Nov 7 2002) reSID v 0.13
- Sidplay 2/w win32 GUI (Jun 2 2005) Libsidplay V2.1.1, ReSID V0.0.2 (sic)
- Sidplay 2, win32 console, V2.0.8 Libsidplay V2.1.0

I tested the following files from the latest HVSC:

Turrican.sid subsong 4 (of 6)
- plays in sidplay1/w, with or without reSID
- silence in sidplay2 and sidplay2/w

Dragon_Breed.sid subsong 14 (of 14)
- plays in sidplay1/w, with or without reSID
- plays in sidplay2 (console)
- silence in sidplay2/w

Peter_Pack_Rat.sid subsong 5 (of 5)
- plays in sidplay1/w, with or without reSID
- plays in sidplay2 (console)
- silence in sidplay2/w