Server crash on GROUP_CONCAT with REPEAT using extremely large length argument

Description

Server crashes (Lost connection, ERROR 2013) when GROUP_CONCAT processes a REPEAT() call with UINT64_MAX as length argument. MCOL-5385 (closed/fixed) addressed RAM accounting in GROUP_CONCAT, but this case still crashes — likely an unchecked allocation before the accounting logic kicks in.

Minimal reproducer

CREATE TABLE t (id INT) ENGINE=Columnstore;

INSERT INTO t VALUES (1);

-- CRASHES server:

SELECT GROUP_CONCAT((SELECT REPEAT('x', 18446744073709551615))) FROM t;

InnoDB returns a truncation warning and empty result, no crash.

Environment

• MariaDB 11.4.11-8, Columnstore 25.10.4 (stable-23.10)
• OS: Ubuntu 24.04

Found by SQL mutation fuzzer.