Ensure an auth instance is available to ListMediaInRoom (#5967)

* Ensure an auth instance is available to ListMediaInRoom

Fixes https://github.com/matrix-org/synapse/issues/5737

* Changelog

changelog.d/5967.bugfix

+Fix list media admin API always returning an error.

synapse/rest/admin/media.py

@@ -60,6 +60,7 @@ class ListMediaInRoom(RestServlet):
 
     def __init__(self, hs):
         self.store = hs.get_datastore()
+        self.auth = hs.get_auth()
 
     @defer.inlineCallbacks
     def on_GET(self, request, room_id):