b=990794 crash on ovrfl in SharedBuffer::Create() r=roc

--HG--
extra : transplant_source : %3F%C2%21%06%C0%A8Z%14%93%82%90%2CT%CE%3E%EC%C2%D0%E4W
b=990794 crash on ovrfl in AllocateAudioBlock r=roc

--HG--
extra : transplant_source : %030X%15H%0DN%AA%7B%AEl%2Bi%2A%AB%3D%9C%23%0C%FF