Bug 917260 - Nullcheck Destination() in AudioContext::DestinationStream. r=ehsan
b=917260 null check mDestination in AudioContext::Shutdown() r=ehsan

This handles the case where Shutdown() is called after Unlink()
(but before the destructor).

--HG--
extra : transplant_source : %97A%BE%60%F5j%F5%F0GT%C2%B6%F8%A2%BD%13%2B%02%83R