KML clustering crash

[hpinkos]

Reported on the forum: https://groups.google.com/forum/?hl=en#!topic/cesium-dev/p1Mv6vpvvnY

If you use this kml file in the Sandcastle clustering example, it crashes. The KML has two placemarks in it

placenames_capital.kml.txt

[pjcozzi]

@bagnell can you please look at this for 1.27 perhaps even at the bug bash?

[mramato]

The fix here appears to be to add defined(entityCluster._labelCollection) && to line 148 of EntityCluster.js, doing so appears to make everything work as expected. I tried (and failed) to create a unit test for this, so perhaps @bagnell can do it when he gets back on Monday.