Gracefully handle ScriptProcessorNode event processing when detached.

Source/modules/webaudio/ScriptProcessorNode.cpp

Index: Source/modules/webaudio/ScriptProcessorNode.cpp
diff --git a/Source/modules/webaudio/ScriptProcessorNode.cpp b/Source/modules/webaudio/ScriptProcessorNode.cpp
index 81088b0dbc21f1f7638a335ce691b62ef6be2b23..b99298045dfd3d9c23e260f3ebe7e4227d02448a 100644
--- a/Source/modules/webaudio/ScriptProcessorNode.cpp
+++ b/Source/modules/webaudio/ScriptProcessorNode.cpp
@@ -226,7 +226,7 @@ void ScriptProcessorNode::process(size_t framesToProcess)
             // 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)));