MDEV-38140: InnoDB index corruption after UPDATE affecting virtual

columns

Issue:
- Purge thread attempts to purge a secondary index record that is not
  delete-marked.

Root Cause:
- When a secondary index includes a virtual column whose v_pos is
  greater than the number of fields in the clustered index record, the
  virtual column is incorrectly skipped while reading from the undo
  record.
- This leads the purge logic to incorrectly assume it is safe to purge
  the secondary index record.
- The code also confuses the nth virtual column with the nth stored
  column when writing ordering columns at the end of the undo record.

Fix:
- In trx_undo_update_rec_get_update(): Skip a virtual column only
  when v_pos == FIL_NULL, not when v_pos is greater than the number
  of fields.
- In trx_undo_page_report_modify(): Ensure ordering columns are
  written based on the correct stored-column positions, without
  confusing them with virtual-column positions.

storage/innobase/trx/trx0rec.cc

@@ -1222,9 +1222,12 @@ store_len:
 				columns that were updated. */
 
 				for (i = 0; i < update->n_fields; i++) {
+					const upd_field_t* fld =
+						upd_get_nth_field(update, i);
+					if (upd_fld_is_virtual_col(fld))
+						continue;
 					const ulint field_no
-						= upd_get_nth_field(update, i)
-						->field_no;
+						= fld->field_no;
 					if (field_no >= index->n_fields
 					    || dict_index_get_nth_field(
 						    index, field_no)->col
@@ -1524,7 +1527,7 @@ trx_undo_update_rec_get_update(
 				&field_no);
 			first_v_col = false;
 			/* This column could be dropped or no longer indexed */
-			if (field_no >= index->n_fields) {
+			if (field_no == FIL_NULL) {
 				/* Mark this is no longer needed */
 				upd_field->field_no = REC_MAX_N_FIELDS;