MDEV-23672: incorrect v_indexes access fix

v_indexes was wrongly copied from the source table.

mysql-test/suite/innodb/t/instant_alter_bugs.test

@@ -446,10 +446,6 @@ create table t1 (
     col_text_g text generated always as (substr(col_text,1,499)) )
 engine innodb row_format = redundant;
 insert into t1 (col_int,col_text) values (0, 'a'), (null, 'b');
-# FIXME: remove the following to trigger the bug
---disable_query_log
-alter table t1 modify column col_text text null, force;
---enable_query_log
 alter table t1 modify column col_text text null, algorithm = instant;
 insert into t1 (col_int,col_text) values (1, null), (null, null);
 update t1 set col_text= 'd';

storage/innobase/handler/handler0alter.cc

@@ -592,6 +592,7 @@ inline bool dict_table_t::instant_column(const dict_table_t& table,
 			mem_heap_alloc(heap, table.n_v_cols * sizeof(*v_cols)));
 		for (ulint i = table.n_v_cols; i--; ) {
 			new (&v_cols[i]) dict_v_col_t(table.v_cols[i]);
+			v_cols[i].v_indexes.clear();
 		}
 	} else {
 		ut_ad(table.n_v_cols == 0);