MDEV-20224: main.derived crashes with ASAN with error use-after-poison

Make sure that the references in the GROUP BY clause that need to be considered for
pushdown from having to where are of Item_field objects

sql/sql_lex.cc

@@ -7902,6 +7902,7 @@ bool st_select_lex::collect_grouping_fields(THD *thd)
     Item *item= *ord->item;
     if (item->type() != Item::FIELD_ITEM &&
         !(item->type() == Item::REF_ITEM &&
+          item->real_type() == Item::FIELD_ITEM &&
         ((((Item_ref *) item)->ref_type() == Item_ref::VIEW_REF) ||
         (((Item_ref *) item)->ref_type() == Item_ref::REF))))
       continue;