MDEV-16353 Server crash on query with CTE

This bug caused crashes for queries with unreferenced non-recursive
CTEs specified by unions.It happened because the function
st_select_lex_unit::prepare() tried to use the value of the field 'derived'
that could not be set for unferenced CTEs as there was no derived
table associated with an unreferenced CTE.

mysql-test/r/cte_nonrecursive.result

@@ -1462,3 +1462,19 @@ a	b
 4	5
 4	3
 DROP TABLE t1;
+#
+# MDEV-16353: unreferenced CTE specified by query with UNION
+#
+with cte as
+(select 1 union select 2 union select 3)
+select 1 as f;
+f
+1
+create table t1 (a int);
+insert into t1 values (2), (1), (7), (1), (4);
+with cte as
+(select * from t1 where a < 2 union select * from t1 where a > 5)
+select 2 as f;
+f
+2
+drop table t1;

mysql-test/t/cte_nonrecursive.test

@@ -1012,3 +1012,21 @@ SELECT a FROM cte;
 WITH cte(a,b) AS (SELECT 4,5 UNION SELECT 4,3) SELECT a,b FROM cte;
 
 DROP TABLE t1;
+
+--echo #
+--echo # MDEV-16353: unreferenced CTE specified by query with UNION
+--echo #
+
+with cte as
+  (select 1 union select 2 union select 3)
+select 1 as f;
+
+create table t1 (a int);
+insert into t1 values (2), (1), (7), (1), (4);
+
+with cte as
+  (select * from t1 where a < 2 union select * from t1 where a > 5)
+select 2 as f;
+
+drop table t1;
+

sql/sql_union.cc

@@ -625,7 +625,7 @@ bool st_select_lex_unit::prepare(THD *thd_arg, select_result *sel_result,
     {
       if (with_element)
       {
-        if (derived->with->rename_columns_of_derived_unit(thd, this))
+        if (with_element->rename_columns_of_derived_unit(thd, this))
 	  goto err; 
         if (check_duplicate_names(thd, sl->item_list, 0))
           goto err;