Click handler called with empty event object on Android

On Android (both native browser and Chrome), the handleTouchEnd_
method is sometimes reached in a state where this.down_ is null.
This check protects against this.click_ being called without
an event object. To see this issue, open any example in Android,
and tap the zoom + or - button.
Merge pull request #811 from ahocevar/conditional-click

Click handler called with empty event object on Android. r=@fredj