Fix crash when opening a bookmark without an open window.
Fix crash when opening a bookmark without an open window.

This fixes a regression appearing in revision 39455 by adding an additional NULL check.

TEST=Open Chromium, close all open windows, open bookmark manager, double click any bookmark. Chromium shouldn't crash.
BUG=37386

Review URL: http://codereview.chromium.org/669078

git-svn-id: http://src.chromium.org/svn/trunk/src@40707 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

Former-commit-id: fa2d9d0c69e90ccd831d52d0b1db2645829e6d34
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