MDEV-21088: Follow-up fix for ROW_FORMAT=REDUNDANT

For ROW_FORMAT=REDUNDANT, we must reserve fixed-length dummy values
for the CHAR columns in the metadata record. This is because in
MariaDB Server 10.4, btr_cur_instant_init_low() will rely on
dict_index_t::trx_id_offset being accurate for the metadata record.

storage/innobase/handler/handler0alter.cc

@@ -4285,7 +4285,7 @@ innobase_add_instant_try(
 				break;
 			case MYSQL_TYPE_STRING:
 				if (col->mbminlen != col->mbmaxlen
-				    || !dict_table_is_comp(user_table)) {
+				    && dict_table_is_comp(user_table)) {
 					goto variable_length;
 				}
 				/* fall through */