Fix issue with "Scheduled" panics when job not scheduled

[ferryvg]

#129 issue

This code are panics:

func One() {}
func Two() {}
func main() {
	scheduler := gocron.NewScheduler()
	scheduler.Every(1).Day().Do(One)
	// and next panics
	exists := scheduler.Scheduled(Two)
	fmt.Println(exists)
}

[JohnRoesler]

hey, this repo is no longer maintained (per the note in the readme) - we have a maintained fork over at https://github.com/go-co-op/gocron and recently added support for this exact use! go-co-op/gocron#106