Tons of crashes on bots after r152425
https://bugs.webkit.org/show_bug.cgi?id=118440
<rdar://problem/14366772>

Reviewed by Sam Weinig.

Null-check the frame's page and early return. It's OK to do this
because it only happens when we're being torn down.

* page/FrameView.cpp:
(WebCore::FrameView::minimumScrollPosition):
(WebCore::FrameView::maximumScrollPosition):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@152434 268f45cc-cd09-0410-ab3c-d52691b4dbfc