Set currentsong to -1 when halting music

This is 2.2 behavior, which I forgot to keep. Otherwise, if music has
halted and you try to play the same track, it simply won't work, because
the current song is the same as the song you're trying to play. This is
what happened with the trinket scripts - the game halted music, then
tried to play the same track.

Fixes #712.

desktop_version/src/Music.cpp

@@ -268,6 +268,7 @@ void musicclass::haltdasmusik(void)
 {
 	/* Just pauses music. This is intended. */
 	pause();
+	currentsong = -1;
 }
 
 void musicclass::silencedasmusik(void)