fix mapbox#8705 actor error after map is removed (mapbox#8711)

If a tile was reloaded shortly before a `map.remove()`, raster tiles
that were in the middle of being loaded would not be aborted leading to
an error when the tried to work with the removed map.

The problem applies to removing and reloading tile.

src/source/source_cache.js

@@ -708,7 +708,7 @@ class SourceCache extends Evented {
         if (tile.uses > 0)
             return;
 
-        if (tile.hasData()) {
+        if (tile.hasData() && tile.state !== 'reloading') {
             this._cache.add(tile.tileID, tile, tile.getExpiryTimeout());
         } else {
             tile.aborted = true;

src/util/dispatcher.js

@@ -2,6 +2,7 @@
 
 import {uniqueId, asyncAll} from './util';
 import Actor from './actor';
+import assert from 'assert';
 
 import type WorkerPool from './worker_pool';
 
@@ -32,12 +33,14 @@ class Dispatcher {
             actor.name = `Worker ${i}`;
             this.actors.push(actor);
         }
+        assert(this.actors.length);
     }
 
     /**
      * Broadcast a message to all Workers.
      */
     broadcast(type: string, data: mixed, cb?: Function) {
+        assert(this.actors.length);
         cb = cb || function () {};
         asyncAll(this.actors, (actor, done) => {
             actor.send(type, data, done);
@@ -49,6 +52,7 @@ class Dispatcher {
      * @returns An actor object backed by a web worker for processing messages.
      */
     getActor(): Actor {
+        assert(this.actors.length);
         this.currentActor = (this.currentActor + 1) % this.actors.length;
         return this.actors[this.currentActor];
     }