slapos_pdm: Fixup in case the instance tree has 2 instances on successor_list

See merge request !604
3 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
failed SlapOS-Master.UnitTest-Master

03:11:31

failed SlapOS.Eggs.UnitTest-Master.Python2

00:18:30

passed SlapOS.Eggs.UnitTest-Master.Python3

00:21:40