webpage needs null check in BackingStore::setCurrentBackingStoreOwner
https://bugs.webkit.org/show_bug.cgi?id=109253

Reviewed by George Staikos.

* Api/BackingStore.cpp:
(BlackBerry::WebKit::BackingStorePrivate::setCurrentBackingStoreOwner):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@142227 268f45cc-cd09-0410-ab3c-d52691b4dbfc