Speculative fix for crash at engaget.com/videos

This is just a symptom of some other oddness in the
fullscreen element stack which was originally worked
around in https://bugs.webkit.org/show_bug.cgi?id=99496
that I accidentally removed as part of
https://codereview.chromium.org/106733004

This adds back the null-check as a workaround until
we can fix fullscreen to not hit this codepath
after the frame or document have detached from the page.

BUG=336239

Review URL: https://codereview.chromium.org/132903014

git-svn-id: svn://svn.chromium.org/blink/trunk@165925 bbb929c8-8fbe-4397-9dbb-9b2b20218538