SIGNAL ... MESSAGE_TEXT accepts malformed strings

This script:

SET NAMES utf8;

SIGNAL SQLSTATE '01000' SET MESSAGE_TEXT='a��b';

SHOW WARNINGS;

returns:

+---------+------+---------+

| Level   | Code | Message |

+---------+------+---------+

| Warning | 1642 | a��b      |

+---------+------+---------+

Note, '��' is a 4-byte UTF-8 character, which is not supported
in MariaDB's 3-byte "utf8" (it requires 4-byte "utf8mb4" to work).

Thus, '��' is a bad sequence of bytes from the point of view "utf8".

It should be fixed either to replace bad byte sequences to question marks,
or just return an error on attempt to set a broken MESSAGE_TEXT.