Add null check to editable in moveCaretSelectionTowardsWindowPoint
https://bugs.webkit.org/show_bug.cgi?id=108962

Patch by David Trainor <dtrainor@chromium.org> on 2013-02-07
Reviewed by Eric Seidel.

* src/WebFrameImpl.cpp:
(WebKit::WebFrameImpl::moveCaretSelectionTowardsWindowPoint):
* tests/WebFrameTest.cpp:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@142093 268f45cc-cd09-0410-ab3c-d52691b4dbfc