In specifying a derived table with a union, for example

CREATE TABLE t (c1 INT KEY,c2 INT,c3 INT) ENGINE=MyISAM;
SELECT * FROM (SELECT * FROM t UNION SELECT * FROM t) AS d (d1,d2);

we bypass an earlier check for the correct number of specified column
names, causing a crash.

Fixed by adding a check for the correct number of supplied arguments
in st_select_lex_unit::rename_types_list()