Null pointer issues seemingly related to race conditions in layer removal

[AndyMoreland]

mapbox-gl-js version: v0.17.0

Steps to Trigger Behavior

1. remove a geojson sourced layer very soon after it was added

Problem: Bucket.create fails while invoking Classes[options.layer.type]: options.layer is undefined in this context.

The stack is:

Bucket.create:mapbox-gl-dev.js:24
unserializeBuckets:mapbox-gl-dev.js:5733
Tile.loadVectorData:mapbox-gl-dev.js:6520
(anonymous function):mapbox-gl-dev.js:4957
Actor.receive:mapbox-gl-dev.js:14657

The problem is that is that data.buckets in Tile.loadVectorData references the layerId of a layer that has already had removeLayer invoked on it, so it's not present in style._layers.

[AndyMoreland]

My quick and dirty fix for my fork is to do:

+        if (input[i].layerId && !style.getLayer(input[i].layerId)) {
+            continue;
+        }
+

in the inner loop of unserializeBuckets in tile.js.