easeTo fails when zoom is undefined or equal to current zoom

In this case `scale` == 1, so [`div(1 - 1 / scale)`](https://github.com/mapbox/mapbox-gl-js/blob/b7639ba6c48007d02dd007a47854f83e14c6db4b/js/ui/easings.js#L200-200) is a divide by zero.

@mourner, can you look? Not sure how to fix this.