Events.removeEventListener fails when no events are registered.

[tjoekbezoer]

Situation
Consider the following code:

var line = new L.Polyline(points, {clickable: false});

Now if you call line.removeEventListener('click') without manually adding a listener, it will throw an error.

Problem
This seems to be caused by the fact that the this['_leaflet_events'] will only exist if addEventListener is called at least once. removeEventListener however does not check the existence of the property.

Solution
Add the check :) See my pull request.