MDEV-17851 Assertion failure srv_undo_tablespaces > 1

trx_assign_rseg_low(): Skip dedicated undo tablespaces if
innodb_undo_tablespaces=0 has been set after the database was created.

storage/innobase/trx/trx0trx.cc

@@ -1002,9 +1002,7 @@ trx_lists_init_at_db_start()
 evenly distributed between 0 and innodb_undo_logs-1
 @return	persistent rollback segment
 @retval	NULL	if innodb_read_only */
-static
-trx_rseg_t*
-trx_assign_rseg_low()
+static trx_rseg_t* trx_assign_rseg_low()
 {
 	if (srv_read_only_mode) {
 		ut_ad(srv_undo_logs == ULONG_UNDEFINED);
@@ -1055,8 +1053,8 @@ trx_assign_rseg_low()
 			ut_ad(rseg->is_persistent());
 
 			if (rseg->space != TRX_SYS_SPACE) {
-				ut_ad(srv_undo_tablespaces > 1);
-				if (rseg->skip_allocation) {
+				if (rseg->skip_allocation
+				    || !srv_undo_tablespaces) {
 					continue;
 				}
 			} else if (trx_rseg_t* next