Handle 0s durations

If you call `countFrom:` with a duration of `0` the label's value is mangled – [along the way](https://github.com/dataxpress/UICountingLabel/blob/26bfd48b39a3e972e61df0730a017c738828cfd9/UICountingLabel.m#L156) a divide-by-zero happens that seems to find its way in.

It should _probably_ be the caller's problem to make sure they aren't sending `0`, but this way they won't have to worry!