Initialize currentsong to -1

There's not any ill effects of it being initialized to 0 that I am aware
of (because in most cases, it either gets overwritten anyways or there
isn't a track playing in the first place), but it shouldn't be 0,
because that's Path Complete, so fixing this just in case.

desktop_version/src/Music.cpp

@@ -737,7 +737,7 @@ musicclass::musicclass(void)
     user_music_volume = USER_VOLUME_MAX;
     user_sound_volume = USER_VOLUME_MAX;
 
-    currentsong = 0;
+    currentsong = -1;
     nicechange = -1;
     nicefade = false;
     quick_fade = true;