DST change causes panic

[codifryed]

Hi there,

I'm using a default Scheduler to trigger jobs at regular intervals, pretty standard, but when the timezone says it's time to move into winter time, aka when the clock moves back an hour, this happens:

thread 'main' panicked at 'called `Option::unwrap()` on a `None` value', .cargo/registry/src/index.crates.io-6f17d22bba15001f/clokwerk-0.4.0/src/intervals.rs:214:41

And it will repeat this error on every restart for the next hour until the "repeated hour" has passed. After that, everything's back to working like normal.

For interval-only use cases I can likely work around this by settings the Scheduler's timezone to UTC, but that of course won't work for everyone.

Thanks for the otherwise great scheduler lib, btw.