MDEV-5356: Server crashes in Item_equal::contains on 2nd execution of a PS

THD::thd->activate_stmt_arena_if_needed() should be used to temporary activating statement arena instead of direct usage of THD::set_n_backup_active_arena() because possible such scenario:
  1) func1 saves current arena and activates copy1 of statement arena
  2) func2 saves copy1 of statement arena setup by func1 and activates copy2
  3) some changes made for copy 2
  4) func2 stores changed copy2 back to statenet arena and activates copy1
  5) func1 store unchanged copy1 back to statemnt arena (rewrite changed copy 2 so changes become lost) and activates arena which was before.

1 files changed, 2 insertions, 5 deletions

diff --git a/sql/sql_select.cc b/sql/sql_select.cc
index d8bbabd5f63..6434c045da0 100644
--- a/sql/sql_select.cc
+++ b/sql/sql_select.cc
@@ -882,11 +882,8 @@ JOIN::optimize()
       MEMROOT for prepared statements and stored procedures.
     */
 
-    Query_arena *arena= thd->stmt_arena, backup;
-    if (arena->is_conventional())
-      arena= 0;                                   // For easier test
-    else
-      thd->set_n_backup_active_arena(arena, &backup);
+    Query_arena *arena, backup;
+    arena= thd->activate_stmt_arena_if_needed(&backup);
 
     sel->first_cond_optimization= 0;