[iOS] avoid WebVideoFullscreenManager crash
https://bugs.webkit.org/show_bug.cgi?id=139852
<rdar://problem/19319302>

Reviewed by Darin Adler.

* WebProcess/ios/WebVideoFullscreenManager.mm:
(WebKit::WebVideoFullscreenManager::didExitFullscreen): NULL-check m_layerHostingContext and m_page.
(WebKit::WebVideoFullscreenManager::setVideoLayerFrameFenced): NULL-check m_layerHostingContext.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@177625 268f45cc-cd09-0410-ab3c-d52691b4dbfc