MDEV-32369: Memory leak when executing PS for query with IN subquery

The memory allocated for an instance of the class Item_direct_ref_to_item
was leaked on second execution of a query run as a prepared statement and
involving conversion of strings with different character sets.

The reason of leaking the memory was caused by the fact that a statement
arena could be already set by the moment the method
Type_std_attributes::agg_item_set_converter() is called.

mysql-test/main/ps_mem_leaks.result

+#
+# MDEV-32369: Memory leak when executing PS for query with IN subquery
+#
+CREATE TABLE t1 (a VARCHAR(10)) ENGINE=MYISAM;
+CREATE TABLE t2 (b VARCHAR(10) CHARACTER SET utf8) ENGINE=MYISAM;
+INSERT INTO t1 VALUES ('b');
+INSERT INTO t2 VALUES ('b');
+PREPARE stmt FROM "SELECT t1.a FROM t1 WHERE t1.a IN (SELECT t2.b FROM t2)";
+EXECUTE stmt;
+a
+b
+EXECUTE stmt;
+a
+b
+DEALLOCATE PREPARE stmt;
+DROP TABLE t1, t2;

mysql-test/main/ps_mem_leaks.test

+# This file contains test cases that use the memory leaks detection feature
+# provided by the cmake build option -DWITH_PROTECT_STATEMENT_MEMROOT
+
+--source include/not_embedded.inc
+# The cmake  option -DWITH_PROTECT_STATEMENT_MEMROOT is used only
+# for debug build
+--source include/have_debug.inc
+
+--echo #
+--echo # MDEV-32369: Memory leak when executing PS for query with IN subquery
+--echo #
+
+CREATE TABLE t1 (a VARCHAR(10)) ENGINE=MYISAM;
+CREATE TABLE t2 (b VARCHAR(10) CHARACTER SET utf8) ENGINE=MYISAM;
+
+INSERT INTO t1 VALUES ('b');
+INSERT INTO t2 VALUES ('b');
+
+PREPARE stmt FROM "SELECT t1.a FROM t1 WHERE t1.a IN (SELECT t2.b FROM t2)";
+
+EXECUTE stmt;
+EXECUTE stmt;
+
+DEALLOCATE PREPARE stmt;
+
+DROP TABLE t1, t2;
+

sql/item.cc

@@ -2584,18 +2584,21 @@ bool Type_std_attributes::agg_item_set_converter(const DTCollation &coll,
     if (conv->fix_fields_if_needed(thd, arg))
       return TRUE;
 
+    if (!thd->stmt_arena->is_conventional())
+    {
       Query_arena *arena, backup;
       arena= thd->activate_stmt_arena_if_needed(&backup);
-    if (arena)
-    {
+
       Item_direct_ref_to_item *ref=
         new (thd->mem_root) Item_direct_ref_to_item(thd, *arg);
       if ((ref == NULL) || ref->fix_fields(thd, (Item **)&ref))
       {
+        if (arena)
           thd->restore_active_arena(arena, &backup);
         return TRUE;
       }
       *arg= ref;
+      if (arena)
         thd->restore_active_arena(arena, &backup);
       ref->change_item(thd, conv);
     }