Fix exception when destroying MediaSourceEngine twice

A new WIP test accidentally destroyed MediaSourceEngine twice, which
should not result in an exception.  This fixes the exception.

Change-Id: I199e3072a3e7994083844c2013ac9c8f76406a63

lib/media/media_source_engine.js

@@ -242,8 +242,11 @@ shaka.media.MediaSourceEngine.prototype.destroy = function() {
     this.video_.load();
   }
 
+  if (this.eventManager_) {
+    cleanup.push(this.eventManager_.destroy());
+  }
+
   return Promise.all(cleanup).then(function() {
-    this.eventManager_.destroy();
     this.eventManager_ = null;
     this.video_ = null;
     this.mediaSource_ = null;