Fix NPE in disconnect path

Fix NPE in disconnect path

We should not be touching instance identifier if the node does
not exist in connection manager. Fixes the following splat:

java.lang.NullPointerException: null
	at org.opendaylight.ovsdb.southbound.OvsdbDataTreeChangeListener.disconnect(OvsdbDataTreeChangeListener.java:151) ~[bundleFile:?]
	at org.opendaylight.ovsdb.southbound.OvsdbDataTreeChangeListener.onDataTreeChanged(OvsdbDataTreeChangeListener.java:103) ~[bundleFile:?]
	at org.opendaylight.mdsal.binding.dom.adapter.BindingDOMDataTreeChangeListenerAdapter.onDataTreeChanged(BindingDOMDataTreeChangeListenerAdapter.java:37) ~[bundleFile:?]
	at org.opendaylight.controller.cluster.datastore.DataTreeChangeListenerActor.dataTreeChanged(DataTreeChangeListenerActor.java:83) [bundleFile:?]
	at org.opendaylight.controller.cluster.datastore.DataTreeChangeListenerActor.handleReceive(DataTreeChangeListenerActor.java:44) [bundleFile:?]

Change-Id: Ibabc7f5324742fce04bc6f02b54a1ead945b15cb
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>