QPID-5785: [Java Broker] Fix NPE when a topic exchange is closed after a message filter has been added to an existing binding.

This corrects a regression introduced by QPID-5709 that was causing a NPE to appear in the test log for QueueBindingTest.

git-svn-id: https://svn.apache.org/repos/asf/qpid/trunk@1603294 13f79535-47bb-0310-9956-ffa450edef68