Protect ZoomBox from division by zero

When the map is at the highest zoom level, the old and the current
resolution will be the same. This would cause a division by zero, which
the JavaScript engine does not recognize as such. So we have to protect
zoomOriginPx from becoming {x: Infinity, y: Infinity}.
Merge pull request #996 from ahocevar/zoombox-infinity

Protect ZoomBox from division by zero (r=@marcjansen)