MDEV-30122 mariabackup.skip_innodb crashes when innodb_undo_tablespaces > 0

- Assign srv_undo_space_id_start when InnoDB opens extra unused
InnoDB undo tablespaces in srv_all_undo_tablespaces_open().
Mariabackup can detect the undo tablespaces using
srv_undo_space_id_start variable

mysql-test/suite/mariabackup/skip_innodb.test

+--source include/innodb_undo_tablespaces.inc
 let $targetdir=$MYSQLTEST_VARDIR/tmp/backup;
 CREATE TABLE t(i int);
 INSERT INTO t VALUES(1);
@@ -9,4 +10,4 @@ exec $XTRABACKUP --prepare --target-dir=$targetdir;
 SELECT * from t;
 DROP TABLE t;
 
-rmdir $targetdir;
\ No newline at end of file
+rmdir $targetdir;

storage/innobase/srv/srv0start.cc

@@ -817,9 +817,11 @@ static dberr_t srv_all_undo_tablespaces_open(bool create_new_undo,
   {
      char name[OS_FILE_MAX_PATH];
      snprintf(name, sizeof name, "%s/undo%03u", srv_undo_dir, i);
-     if (!srv_undo_tablespace_open(create_new_undo, name, i))
+     uint32_t space_id= srv_undo_tablespace_open(create_new_undo, name, i);
+     if (!space_id)
        break;
-     ++srv_undo_tablespaces_open;
+     if (0 == srv_undo_tablespaces_open++)
+       srv_undo_space_id_start= space_id;
   }
 
   return DB_SUCCESS;