Oilpan: Don't destruct HRTFDatabaseLoader::m_thread during a sweeping phase

Source/modules/webaudio/AudioListener.cpp

 /*
  * Copyright (C) 2010 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  *
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
  * 2.  Redistributions in binary form must reproduce the above copyright
@@ skipping 74 matching lines @@
         m_hrtfDatabaseLoader = HRTFDatabaseLoader::createAndLoadAsynchronouslyIfNecessary(sampleRate);
 }
 
 bool AudioListener::isHRTFDatabaseLoaded()
 {
     return m_hrtfDatabaseLoader->isLoaded();
 }
 
 void AudioListener::waitForHRTFDatabaseLoaderThreadCompletion()
 {
-    m_hrtfDatabaseLoader->waitForLoaderThreadCompletion();
+    if (m_hrtfDatabaseLoader)
+        m_hrtfDatabaseLoader->waitForLoaderThreadCompletion();
 }
 
 void AudioListener::markPannersAsDirty(unsigned type)
 {
     for (unsigned i = 0; i < m_panners.size(); ++i)
         m_panners[i]->markPannerAsDirty(type);
 }
 
 void AudioListener::setPosition(const FloatPoint3D &position)
 {
@@ skipping 57 matching lines @@
 
     // This synchronizes with panner's process().
     MutexLocker listenerLocker(m_listenerLock);
     m_speedOfSound = speedOfSound;
     markPannersAsDirty(PannerNode::DopplerRateDirty);
 }
 
 } // namespace blink
 
 #endif // ENABLE(WEB_AUDIO)