Server crashes in Static_binary_string::length

Description
Using `PERCENTILE_DISC` window function with specific ordering triggers a crash.

Reproduction Steps
CREATE DATABASE IF NOT EXISTS HuangHe;
USE HuangHe;
CREATE TABLE t (a varchar(8));
INSERT INTO t VALUES ('foo'), (NULL);
SELECT PERCENTILE_DISC(2) WITHIN GROUP (ORDER BY a) OVER () FROM t;
DROP TABLE t;