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.

sql/sql_base.cc

@@ -7843,12 +7843,9 @@ bool setup_tables(THD *thd, Name_resolution_context *context,
     {
       DBUG_ASSERT(table_list->view &&
                   table_list->effective_algorithm == VIEW_ALGORITHM_MERGE);
-      Query_arena *arena= thd->stmt_arena, backup;
+      Query_arena *arena, backup;
+      arena= thd->activate_stmt_arena_if_needed(&backup);
       bool res;
-      if (arena->is_conventional())
-        arena= 0;                                   // For easier test
-      else
-        thd->set_n_backup_active_arena(arena, &backup);
       res= table_list->setup_underlying(thd);
       if (arena)
         thd->restore_active_arena(arena, &backup);

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;
 

sql/sql_view.cc

@@ -1121,11 +1121,7 @@ bool mysql_make_view(THD *thd, File_parser *parser, TABLE_LIST *table,
     will be TRUE as far as we make new table cache).
   */
   old_lex= thd->lex;
-  arena= thd->stmt_arena;
-  if (arena->is_conventional())
-    arena= 0;
-  else
-    thd->set_n_backup_active_arena(arena, &backup);
+  arena= thd->activate_stmt_arena_if_needed(&backup);
 
   /* init timestamp */
   if (!table->timestamp.str)