Agent crash @ EndofRibTx
Agent crash @ EndofRibTx

Peer walk is started to send of end of rib to CN.
Before this walk is over peer goes down, resulting in releasing xmpp channel.
Also with peer down, request is enqueued to stop the walks.
However before stop walk request is processed, walk done gets called which
triggers EndOfRibTx. Here xmpp channel is accessed which is freed.

Solution:
NULL check.

Change-Id: I09fdd6cdf60f3147e78031b107b237b7be415e1c
Closes-bug: #1690762

nischalsheth/contrail-controller,eonpatapon/contrail-controller,rombie/contrail-controller,eonpatapon/contrail-controller,rombie/contrail-controller,nischalsheth/contrail-controller,nischalsheth/contrail-controller,rombie/contrail-controller,eonpatapon/contrail-controller,nischalsheth/contrail-controller,nischalsheth/contrail-controller,eonpatapon/contrail-controller,rombie/contrail-controller,nischalsheth/contrail-controller,eonpatapon/contrail-controller,eonpatapon/contrail-controller,rombie/contrail-controller,rombie/contrail-controller,nischalsheth/contrail-controller,nischalsheth/contrail-controller,rombie/contrail-controller,eonpatapon/contrail-controller