--------------
CREATE PROCEDURE t2p(IN p INT)
BEGIN
    IF p <> 1 THEN
        RETURN;                       
    END IF;
    SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = 'x';
END
--------------

ERROR 1064 (42000) at line 3: You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near ';                       
    END IF;
    SIGNAL SQLSTATE '45000' SET MESSAGE_...' at line 4