NULL check the return value during navigation to prevent a crash.
NULL check the return value during navigation to prevent a crash.

BUG=1384003
Review URL: http://codereview.chromium.org/2966

git-svn-id: http://src.chromium.org/svn/trunk/src@2366 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

Former-commit-id: 754d108a8554ea138be3aa1d18c53dda0785576e
meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser