Fix parenthesis in sanitizeCoordinates()

The current parenthesis can cause exceptions.
Because due to the lack of parenthesis, if ```offsetX = 0``` then the whole expression is evaluated to true.
And in a desktop browser the ```event_.changedTouches``` is undefined, however we're trying to use 
```event.event_.changedTouches[0]``` inside the if block which causes an exception.

This PR fixes this problem.