db/DatabaseSong: make the Storage optional

Some database plugins don't use a Storage (e.g. UPnP), and with this
plugin, DatabaseDetachSong() can crash.

src/SongLoader.cxx

@@ -41,7 +41,7 @@ SongLoader::LoadFromDatabase(const char *uri) const
 {
 #ifdef ENABLE_DATABASE
 	if (db != nullptr)
-		return DatabaseDetachSong(*db, *storage, uri);
+		return DatabaseDetachSong(*db, storage, uri);
 #else
 	(void)uri;
 #endif