HRTFElevation segfault if a null AudioBus is returned by loadPlatformResource
https://bugs.webkit.org/show_bug.cgi?id=108504

Patch by Jesus Sanchez-Palencia <jesus.palencia@openbossa.org> on 2013-02-01
Reviewed by Kenneth Russell.

Fixed a segfault by checking if the returned AudioBus is null or not.
If it is, we should early return in the same way it was being done already
in HRTFElevation::calculateKernelsForAzimuthElevation();

No new tests, covered by existing tests.

* platform/audio/HRTFElevation.cpp:
(WebCore::getConcatenatedImpulseResponsesForSubject):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@141611 268f45cc-cd09-0410-ab3c-d52691b4dbfc