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: dd90618784b6a4b323ea0c23a071cb1c9e6f2ac7@40707 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser