playlist: exclude end of range

In a range "start:end", "end" itself should not be included.  Use the
same semantics as other languages implementing ranges, e.g. Python.

src/command.c

@@ -390,7 +390,7 @@ handle_currentsong(struct client *client,
 	if (song < 0)
 		return COMMAND_RETURN_OK;
 
-	result = playlistInfo(client, song, song);
+	result = playlistInfo(client, song, song + 1);
 	return print_playlist_result(client, result);
 }