MDEV-21341: Fix optimizer-related UBSAN failures, part #1:

Fix wrong typecast

sql/sql_select.cc

@@ -14157,7 +14157,7 @@ static COND* substitute_for_best_equal_field(THD *thd, JOIN_TAB *context_tab,
     }	 
   }
   else if (cond->type() == Item::FUNC_ITEM && 
-           ((Item_cond*) cond)->functype() == Item_func::MULT_EQUAL_FUNC)
+           ((Item_func*) cond)->functype() == Item_func::MULT_EQUAL_FUNC)
   {
     item_equal= (Item_equal *) cond;
     item_equal->sort(&compare_fields_by_table_order, table_join_idx);