Music: Do not do fades if not playing

This fixes a regression where entering playtesting while a track was
fading out (by exiting out of playtesting with a track playing and then
immediately entering back in with the level start music set) would
result in no music.

The cause is the game doing fades even though nothing is playing, which
puts it in a confusing state.

desktop_version/src/Music.cpp

@@ -357,6 +357,11 @@ static enum FadeCode processmusicfade(struct FadeState* state, int* volume)
 
 void musicclass::fadeMusicVolumeIn(int ms)
 {
+    if (halted())
+    {
+        return;
+    }
+
     m_doFadeInVol = true;
     m_doFadeOutVol = false;
 
@@ -374,6 +379,11 @@ void musicclass::fadeMusicVolumeIn(int ms)
 
 void musicclass::fadeMusicVolumeOut(const int fadeout_ms)
 {
+    if (halted())
+    {
+        return;
+    }
+
     m_doFadeInVol = false;
     m_doFadeOutVol = true;