Destroy the FxMixer before the Mixer when shutting down

Fixes #2584.

src/core/Engine.cpp

@@ -94,8 +94,8 @@ void LmmsCore::destroy()
 	deleteHelper( &s_bbTrackContainer );
 	deleteHelper( &s_dummyTC );
 
-	deleteHelper( &s_mixer );
 	deleteHelper( &s_fxMixer );
+	deleteHelper( &s_mixer );
 
 	deleteHelper( &s_ladspaManager );