Initialize nicechange to -1 for consistency

The game uses -1 to denote "no song" elsewhere, especially when a
niceplay() has finished processing and it sets nicechange to -1 there,
too.

desktop_version/src/Music.cpp

@@ -136,7 +136,7 @@ void musicclass::init()
 	FadeVolAmountPerFrame = 0;
 
 	currentsong = 0;
-	nicechange = 0;
+	nicechange = -1;
 	nicefade = false;
 	resumesong = 0;
 	fadeoutqueuesong = -1;