Initialize stamped and ended, implementing the small optimization todo

[yuwash]

The lack of initialization of the stamped variable was causing a failure from an unexpected place when running it with a faulty ical that lacked a BEGIN:V… before having the first DTSTAMP. Probably the same with ended. Only affects bad strings, so it’s more of an error handling improvement…

[tobixen]

Thanks for the contribution. Unfortunately I don't have time to do a code review right now, but possibly later today or tomorrow :-)

[tobixen]

It would be nice with ...

1. test code proving that the current code is breaking

2. unit test proving that the class does what it's intended to do

[tobixen]

And 3 also, updates to the CHANGELOG.md-file. Anyway, if this actually solves a bug, I suppose it's best to just merge it now and indicate in a separate issue that we need tests.