SongSave: return DetachedSong, not a std::unique_ptr<>

Eliminate unnecessary dynamic allocations.

src/db/plugins/simple/DirectorySave.cxx

@@ -165,7 +165,7 @@ directory_load(TextFile &file, Directory &directory)
 			auto detached_song = song_load(file, name,
 						       &audio_format);
 
-			auto song = std::make_unique<Song>(std::move(*detached_song),
+			auto song = std::make_unique<Song>(std::move(detached_song),
 							   directory);
 			song->audio_format = audio_format;