Fix NullPointerException trying to clear pending messages and flows

Author: marciogranzotto

org.eclipse.paho.client.mqttv3/src/main/java/org/eclipse/paho/client/mqttv3/internal/ClientState.java

@@ -1366,8 +1366,12 @@ public void quiesce(long timeout) {
 			// Quiesce time up or inflight messages delivered.  Ensure pending delivery
 			// vectors are cleared ready for disconnect to be sent as the final flow.
 			synchronized (queueLock) {
-				pendingMessages.clear();				
-				pendingFlows.clear();
+				if (pendingMessages != null) {
+					pendingMessages.clear();
+				}
+				if (pendingFlows != null) {
+					pendingFlows.clear();
+				}
 				quiescing = false;
 				actualInFlight = 0;
 			}