xds: Check for validity of xdsClient in ClusterImplLbHelper (grpc#11553)

* Added null check for xdsClient in onSubChannelState. This avoids NPE
for xdsClient when LB is shutdown and onSubChannelState is called later
as part of listener callback. As shutdown is racy and eventually consistent,
this check would avoid calculating locality after LB is shutdown.

xds/src/main/java/io/grpc/xds/ClusterImplLoadBalancer.java

@@ -243,7 +243,8 @@ public void start(SubchannelStateListener listener) {
           delegate().start(new SubchannelStateListener() {
             @Override
             public void onSubchannelState(ConnectivityStateInfo newState) {
-              if (newState.getState().equals(ConnectivityState.READY)) {
+              // Do nothing if LB has been shutdown
+              if (xdsClient != null && newState.getState().equals(ConnectivityState.READY)) {
                 // Get locality based on the connected address attributes
                 ClusterLocality updatedClusterLocality = createClusterLocalityFromAttributes(
                     subchannel.getConnectedAddressAttributes());