MUC: Fix traceback on muc#admin query with missing <item> child (#1242)

Caused by a missing return statement introduced in 494938dec5d8

plugins/muc/muc.lib.lua

@@ -904,6 +904,7 @@ function room_mt:handle_admin_query_set_command(origin, stanza)
 	local item = stanza.tags[1].tags[1];
 	if not item then
 		origin.send(st.error_reply(stanza, "cancel", "bad-request"));
+		return true;
 	end
 	if item.attr.jid then -- Validate provided JID
 		item.attr.jid = jid_prep(item.attr.jid);