Tweak addEvent so it silently ignores non-eventing contexts

I was trying to use a recent QUnit within Rhino (for https://github.com/finnjohnsen/jstest), but couldn't, since the simple context set up in Rhino does not have events, and a very simple 'root' context object.

This tiny change made 1.12.0 usable for my purpose, but it could be that I'm just doing it wrong.