Fix Bug 444318 - Check null when calling MqttAndroidClient#isConnected

Change-Id: Id698850b10e0707a195e279150c1483021e0f15e
Signed-off-by: zyang5e4 <zyyangbj@cn.ibm.com>

Author: zyyangbj

org.eclipse.paho.android.service/org.eclipse.paho.android.service/src/org/eclipse/paho/android/service/MqttAndroidClient.java

@@ -225,7 +225,10 @@ public MqttAndroidClient(Context context, String serverURI,
 	 */
 	@Override
 	public boolean isConnected() {
-		return mqttService.isConnected(clientHandle);
+		if (mqttService != null)
+			return mqttService.isConnected(clientHandle);
+		else 
+			return false;
 	}
 
 	 /**