Gracefully handle ScriptProcessorNode event processing when detached.

Source/modules/webaudio/ScriptProcessorNode.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
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 208 matching lines @@
     // When this happens, fire an event and swap buffers.
     if (!m_bufferReadWriteIndex) {
         // Avoid building up requests on the main thread to fire process events when they're not being handled.
         // This could be a problem if the main thread is very busy doing other things and is being held up handling previous requests.
         // The audio thread can't block on this lock, so we call tryLock() instead.
         MutexTryLocker tryLocker(m_processEventLock);
         if (!tryLocker.locked()) {
             // We're late in handling the previous request. The main thread must be very busy.
             // The best we can do is clear out the buffer ourself here.
             outputBuffer->zero();
-        } else {
+        } else if (context()->executionContext()) {
             // Fire the event on the main thread, not this one (which is the realtime audio thread).
             m_doubleBufferIndexForEvent = m_doubleBufferIndex;
             context()->executionContext()->postTask(createCrossThreadTask(&ScriptProcessorNode::fireProcessEvent, PassRefPtrWillBeRawPtr<ScriptProcessorNode>(this)));
         }
 
         swapBuffers();
     }
 }
 
 void ScriptProcessorNode::fireProcessEvent()
@@ skipping 38 matching lines @@
 void ScriptProcessorNode::trace(Visitor* visitor)
 {
     visitor->trace(m_inputBuffers);
     visitor->trace(m_outputBuffers);
     AudioNode::trace(visitor);
 }
 
 } // namespace WebCore
 
 #endif // ENABLE(WEB_AUDIO)