http2: fix memory leak on nghttp2 hd threshold

PR-URL: nodejs#41502
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Stephen Belanger <admin@stephenbelanger.com>

test/parallel/test-http2-options-max-headers-exceeds-nghttp2.js

@@ -43,4 +43,4 @@ server.listen(0, common.mustCall(() => {
     message: 'Session closed with error code 9'
   }));
   req.end();
-}));
+}));