Heap-buffer-overflow in WebCore::AudioBufferSourceNode::process
https://bugs.webkit.org/show_bug.cgi?id=108515

After calling setBuffer() with a buffer having a different number of
channels, there can in rare cases be a slight delay before the output
bus is updated to the new number of channels because of use of
tryLocks() in the context's updating system.
In this case, if the the buffer has just been changed and we're
not quite ready yet then just output silence.

Reviewed by Chris Rogers.

* Modules/webaudio/AudioBufferSourceNode.cpp:
(WebCore::AudioBufferSourceNode::process):
(WebCore::AudioBufferSourceNode::renderFromBuffer):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@141851 268f45cc-cd09-0410-ab3c-d52691b4dbfc