fix for infinite route loop which is triggered by muc_light

the loop is triggered when someone sends special affiliation change stanza.
When someone tries to invite a user to a room and the user is wrongly named in
the invitation stanza (muclight.localhost domain instead of localhost), the
infinite loop of ejabberd_router:route/3 fun will be called.

apps/ejabberd/src/mod_muc_light.erl

@@ -229,6 +229,8 @@ process_packet(From, #jid{ luser = RoomU } = To, {ok, RequestToRoom}, OrigPacket
 process_packet(From, To, {error, _} = Err, OrigPacket) ->
     mod_muc_light_codec_backend:encode_error(
       Err, From, To, OrigPacket, fun ejabberd_router:route/3);
+process_packet(_From, _To, ignore, _OrigPacket) ->
+     ok;
 process_packet(From, To, _InvalidReq, OrigPacket) ->
     mod_muc_light_codec_backend:encode_error(
       {error, bad_request}, From, To, OrigPacket, fun ejabberd_router:route/3).