Modify interaction breaks mouse wheel zoom

In the [modify-features example](http://ol3js.org/en/master/examples/modify-features.html), the mouse wheel zoom is unresponsive when the mouse is near or over a selected feature.

To reproduce:
* Open the example
* Click on a country
* Without moving the mouse, try to zoom the map with mouse wheel

Mouse wheel zoom does not work whenever the guideline vertex is drawn. This happens when the mouse is near a selected country.

(Tested on Chrome / Mac OS X)