Fix overzealous kicking of guest users (#2453)

We should only kick guest users if the guest access event is authorised.

synapse/handlers/federation.py

@@ -1606,7 +1606,7 @@ class FederationHandler(BaseHandler):
 
             context.rejected = RejectedReason.AUTH_ERROR
 
-        if event.type == EventTypes.GuestAccess:
+        if event.type == EventTypes.GuestAccess and not context.rejected:
             yield self.maybe_kick_guest_users(event)
 
         defer.returnValue(context)