trx_undo_page_report_modify(): Move an over-zealous assertion

Apparently, the first fields of a clustered index may be updated
in an update_undo vector when the index is ID_IND of SYS_FOREIGN,
as part of renaming the table during ALTER TABLE.
Normally, updates of the PRIMARY KEY should be logged as
delete-mark and an insert.

storage/innobase/trx/trx0rec.cc

@@ -1115,9 +1115,8 @@ trx_undo_page_report_modify(
 				goto store_len;
 			}
 
-			ut_ad(pos >= index->first_user_field());
-
 			if (UNIV_UNLIKELY(update->is_metadata())) {
+				ut_ad(pos >= index->first_user_field());
 				ut_ad(rec_is_metadata(rec, index));
 
 				if (rec_is_alter_metadata(rec, index)) {