address lua warning where id could be `nil

src/util/libbl.lua

@@ -536,6 +536,9 @@ function bl.schedule(time, cb, ...)
 end
 function _bllua_schedule_callback(id)
 	id = tonumber(id)
+	if id == nil then
+		error('_ts_schedule_callback: invalid id')
+	end
 	local sch = bl._scheduleTable[id]
 	if not sch then error('_ts_schedule_callback: no schedule with id '..id) end
 	bl._scheduleTable[id] = nil