Stub I_UnloadSong because we already unload in I_StopMusic

* Stop-gap for now. Ideally the logic would be in the respective places.

# Conflicts:
#	src/sdl/mixer_sound.c

src/sdl/mixer_sound.c

@@ -705,8 +705,11 @@ boolean I_LoadSong(char *data, size_t len)
 
 void I_UnloadSong(void)
 {
-	Mix_FreeMusic(music);
-	music = NULL;
+	// \todo unhook looper
+	//var_cleanup();
+	//Mix_FreeMusic(music);
+	//music = NULL;
+	I_StopSong();
 }
 
 boolean I_PlaySong(boolean looping)