[Mac] media engine may deliver NULL in-band "cue"
https://bugs.webkit.org/show_bug.cgi?id=116180

Reviewed by Jer Noble.

* platform/graphics/avfoundation/InbandTextTrackPrivateAVF.cpp:
(WebCore::InbandTextTrackPrivateAVF::processCue): NULL check before logging cue count.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@150153 268f45cc-cd09-0410-ab3c-d52691b4dbfc