Querying I_S.PARAMETERS can crash with a corrupted mysql.proc

I intentionally break mysql.proc by adding an extra value into the type column, and update type of an existing routine to the new bad value. After this change, a query to I_S.PARAMETERS crashes the server.

CREATE OR REPLACE FUNCTION f1 (a INT) RETURNS INT RETURN 10;

ALTER TABLE mysql.proc MODIFY type enum('FUNCTION','PROCEDURE','PACKAGE','PACKAGE BODY','XXX');

UPDATE mysql.proc SET type='XXX' WHERE name='f1';

SELECT * FROM INFORMATION_SCHEMA.PARAMETERS;

This problem was introduced by the patch for MDEV-13415.