GPX: Invoke _handleLayerMouseOver on the correct layer

[adg29]

Added the conditional typeof layer==='undefined'. This ensures that layer is correctly assigned to the line object that is made available by the addLine callback in the GPX use-case. Previously there was only a check for null, which prevented the correct assignment to the layer that is assigned the mouseover.

With the correct assignment, mouseover is now triggered on the line and the corresponding point in the elevation diagram is indicated.

Tested on a GPX file @ http://codepen.io/adg29/full/jAzpRo/