Fix the broken test suite in -debug build.

sql/sql_select.cc:
  If we use subqueries, we can have double-free of tmp_table_param.copy_field
  in JOIN::destroy and in JOIN::join_free because.

sql/sql_select.cc

@@ -5986,6 +5986,8 @@ void JOIN::cleanup(bool full)
   */
   if (full)
   {
+    if (tmp_join)
+      tmp_table_param.copy_field= 0;
     group_fields.delete_elements();
     /*
       We can't call delete_elements() on copy_funcs as this will cause