MDEV-30828 Prevent pushing down unions with incorrect ORDER BY

Fake_select_lex->join was prepared at the unit execution stage so
the validation of fake_select_lex before the unit pushdown
was incomplete. That caused pushing down of statements having
an incorrect ORDER BY clause.
This commit moves preparation of the fake_select_lex->join to the unit
prepare() method, before initializing of the pushdown handler,
so incorrect clauses error out before being pushed down

sql/sql_select.h

@@ -1596,6 +1596,8 @@ public:
   */
   bool impossible_where; 
 
+  bool prepared;
+
   /*
     All fields used in the query processing.