bug 1198656 interpret null ConvolverNode mBuffer as a buffer of zeros r=padenot

Zero output even when normalize is set is consistent with
http://webaudio.github.io/web-audio-api/#widl-ConvolverNode-normalize
due to the specified isinf() test.

--HG--
extra : transplant_source : %17%BA%82%80%7DK%0EZOGg%7B%C0%0D%BF%93%D2%82%A0%E8

dom/media/webaudio/ConvolverNode.cpp

@@ -107,7 +107,7 @@ public:
                             bool* aFinished) override
   {
     if (!mReverb) {
-      *aOutput = aInput;
+      aOutput->SetNull(WEBAUDIO_BLOCK_SIZE);
       return;
     }