Crash when move my finger outside the canvas during the drawing

[ericcheungwork]

Crash on this line:

@zengxinzhy do you have any idea on that? thanks!!

[dsmailes]

I've opened a PR that fixes this. If you want to do it in your local branch just change this code in Utils.swift:

func abort() { if !self.times.isEmpty { self.times.removeLast() } }