Fix for crash when a bookmark URL is an empty string

BUG=19852
TEST=See repro steps of the bug 

Patch by Bruno Calvignac <BrunoCalvignac@gmail.com>
Original CL http://codereview.chromium.org/282004

Review URL: http://codereview.chromium.org/342046

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@30540 0039d316-1c4b-4281-b951-d872f2087c98