[Mac] guard against NULL languages array
https://bugs.webkit.org/show_bug.cgi?id=109595

Reviewed by Dean Jackson.

No new tests, existing tests won't crash if this is correct.

* page/CaptionUserPreferencesMac.mm:
(WebCore::CaptionUserPreferencesMac::preferredLanguages):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@142639 268f45cc-cd09-0410-ab3c-d52691b4dbfc