NaN error when touch point is the same as the centre of the circle.

[mikemonteith]

If the user's drag coordinate is at the centre of the CircularSlider, a crash occurs because of a "divide by zero" issue.

[behrad-kzm]

in function moveHandle() :

    func moveHandle(lastPoint:CGPoint){       
       //Get the center
        let centerPoint:CGPoint  = CGPointMake(self.frame.size.width/2, self.frame.size.height/2);
        //Calculate the direction from a center point and a arbitrary position.
       //bug fix:
        var currentAngle:Double = AngleFromNorth(centerPoint, p2: lastPoint, flipped: false);
      if currentAngle.isNaN {
        currentAngle = 0
      }
        let angleInt = Int(floor(currentAngle))

        //Store the new angle
        angle = Int(360 - angleInt)

        //Update the textfield
        textField!.text = "\(angle)"

        //Redraw
        setNeedsDisplay()

    }