mapper: ignore duplicate slashes in playlists

When there are duplicate slashes in the song paths, eliminate them;
example:

 /var/lib/mpd/music//foo.mp3

becomes:

 /var/lib/mpd/music/foo.mp3

The slash is only detected at the border between the music_directory
and the local part.

src/mapper.c

@@ -205,6 +205,9 @@ map_fs_to_utf8(const char *path_fs)
 		/* not within musicDir */
 		return NULL;
 
+	while (path_fs[0] == G_DIR_SEPARATOR)
+		++path_fs;
+
 	return fs_charset_to_utf8(path_fs);
 }