song: song_get_url: fix "/" prefix for songs in music_root

src/song.c

@@ -124,7 +124,7 @@ song_get_url(struct song *song, char *path_max_tmp)
 	assert(song != NULL);
 	assert(*song->url);
 
-	if (!song->parent || !song->parent->path)
+	if (!song->parent || isRootDirectory(song->parent->path))
 		strcpy(path_max_tmp, song->url);
 	else
 		pfx_dir(path_max_tmp, song->url, strlen(song->url),