MDEV-17823 Assertion failed when accessing indexed instantly added column

This assertion would fail when a secondary index record for an
instantly added column was accessed.

It is unclear to me why this code path is executed so rarely.
I was unable to cover it even when using FORCE INDEX.

row_sel_sec_rec_is_for_clust_rec(): Remove the assertion, and use
the proper function rec_get_nth_cfield().

row_sel_store_mysql_field_func(): Simply use rec_get_nth_cfield()
instead of duplicating its logic.

storage/innobase/row/row0sel.cc

@@ -254,9 +254,9 @@ row_sel_sec_rec_is_for_clust_rec(
 			clust_field = static_cast<byte*>(vfield->data);
 		} else {
 			clust_pos = dict_col_get_clust_pos(col, clust_index);
-			ut_ad(!rec_offs_nth_default(clust_offs, clust_pos));
-			clust_field = rec_get_nth_field(
-				clust_rec, clust_offs, clust_pos, &clust_len);
+			clust_field = rec_get_nth_cfield(
+				clust_rec, clust_index, clust_offs,
+				clust_pos, &clust_len);
 		}
 
 		sec_field = rec_get_nth_field(sec_rec, sec_offs, i, &sec_len);
@@ -3010,17 +3010,7 @@ row_sel_store_mysql_field_func(
 	} else {
 		/* The field is stored in the index record, or
 		in the metadata for instant ADD COLUMN. */
-
-		if (rec_offs_nth_default(offsets, field_no)) {
-			ut_ad(dict_index_is_clust(index));
-			ut_ad(index->is_instant());
-			const dict_index_t* clust_index
-				= dict_table_get_first_index(prebuilt->table);
-			ut_ad(index == clust_index);
-			data = clust_index->instant_field_value(field_no,&len);
-		} else {
-			data = rec_get_nth_field(rec, offsets, field_no, &len);
-		}
+		data = rec_get_nth_cfield(rec, index, offsets, field_no, &len);
 
 		if (len == UNIV_SQL_NULL) {
 			/* MySQL assumes that the field for an SQL