fix bug for app crashed when cleansession is set to true

Change-Id: I865d42a979c8e76398b71ee6bc51a9345c96c004
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/DatabaseMessageStore.java

@@ -350,6 +350,9 @@ protected void finalize() throws Throwable {
 	 */
 	@Override
 	public void clearArrivedMessages(String clientHandle) {
+		
+		db = mqttDb.getWritableDatabase();
+		
 		int rows = 0;
 		if (clientHandle == null) {
 			traceHandler.traceDebug(TAG,