predict: fully deactivate prediction with `period = 0`

That setting is recommended by documentation but it would still leave the timer triggering repeatedly. Maybe it didn't cause any practical difference, but it was unnecessary and possibly confusing.

modules/predict/predict.lua

@@ -140,7 +140,7 @@ function predict.process()
 end
 
 function predict.init()
-	if predict.window > 0 then
+	if predict.window > 0 and predict.period > 0 then
 		predict.current_epoch = predict.epoch()
 		predict.ev_sample = event.after(next_event(), predict.process)
 	end