fix(nullRef): nullify reference to the destroyed slider

[VinceOPS]

Hi ionic team.

I've got an issue today with ionSlides, using it in a ui-view. When leaving the view containing the slider, by loading another one into the ui-view, the update method of ionSlides was triggered and executed _this.__slider.update() in spite of the "falsy-check" just before. Indeed, the instance of the slider is kept even after the slider is destroyed (because of $scope $destroy event).

A simple PR to nullify the reference after the slider is destroyed.

In order for this piece of code to work properly:

  this.update = function() {
    $timeout(function() {
      if (!_this.__slider) {
        return;
      }

      // not called if the reference is properly nullified
      _this.__slider.update();
      [...]

[adamdbradley]

thanks!