Fix regression with music fade transitions not working

Unfortunately, 1de459d9b4 caused a
regression where musicclass::niceplay() didn't work, because fading out
the music would cause haltdasmusik() to be called, which would reset the
fade variables.

To fix this, haltdasmusik() will now only reset the fade variables if
it's not called from a fade, which is signaled with a function
parameter.

desktop_version/src/Music.cpp

@@ -1026,14 +1026,22 @@ void musicclass::pause(void)
 }
 
 void musicclass::haltdasmusik(void)
+{
+    haltdasmusik(false);
+}
+
+void musicclass::haltdasmusik(const bool from_fade)
 {
     /* Just pauses music. This is intended. */
     pause();
     currentsong = -1;
     m_doFadeInVol = false;
     m_doFadeOutVol = false;
-    nicefade = false;
-    nicechange = -1;
+    if (!from_fade)
+    {
+        nicefade = false;
+        nicechange = -1;
+    }
 }
 
 void musicclass::silencedasmusik(void)
@@ -1145,7 +1153,7 @@ void musicclass::processmusicfadeout(void)
     {
         musicVolume = 0;
         m_doFadeOutVol = false;
-        haltdasmusik();
+        haltdasmusik(true);
     }
 }