fix warning, it is not unused

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1641984 13f79535-47bb-0310-9956-ffa450edef68

lucene/test-framework/src/java/org/apache/lucene/mockfile/FilterFileSystem.java

@@ -74,6 +74,7 @@ public class FilterFileSystem extends FileSystem {
       parent.onClose();
     } else {
       try (FileSystem d = delegate) {
+        assert d != null; // avoid stupid compiler warning
         parent.onClose();
       }
     }