Fix Bug 440698 - thread blocked in MqttClient.disconnect()

Signed-off-by: bzhangxmt <zhbinbj@cn.ibm.com>

Author: bzhangxmt

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

@@ -524,6 +524,14 @@ protected void undo(MqttPublish message) throws MqttPersistenceException {
 	 */
 	public MqttToken checkForActivity() throws MqttException {
 		final String methodName = "checkForActivity";
+		
+        synchronized (quiesceLock) {
+            // ref bug: https://bugs.eclipse.org/bugs/show_bug.cgi?id=440698
+            // No ping while quiescing
+            if (quiescing) {
+                return null;
+            }
+        }
 
 		MqttToken token = null;
 		long nextPingTime = getKeepAlive();