master: fix crash of backup master when disconnected from upstream while serving clients

This fixes:

  Traceback (most recent call last):
    File "neo/master/app.py", line 172, in run
      self._run()
    File "neo/master/app.py", line 182, in _run
      self.playPrimaryRole()
    File "neo/master/app.py", line 314, in playPrimaryRole
      self.backup_app.provideService())
    File "neo/master/backup_app.py", line 101, in provideService
      app.changeClusterState(ClusterStates.STARTING_BACKUP)
    File "neo/master/app.py", line 474, in changeClusterState
      ) or not node.isClient(), (state, node)
  AssertionError: (<EnumItem STARTING_BACKUP (4)>, <ClientNode(uuid=C1, state=RUNNING, connection=<ServerConnection(nid=C1, address=127.0.0.1:52430, handler=ClientReadOnlyServiceHandler, fd=59, on_close=onConnectionClosed, server) at 7f38f5628390>) at 7f38f5628ad0>)