Jetpack: When calling Jetpack::get_content_width() if the global content_width isn't set, pass a zero to the filter instead.  Themes really should be setting this on their own, however.  Fixes #1878

git-svn-id: https://plugins.svn.wordpress.org/jetpack/trunk@758489 b8457f37-d9ea-0310-8a92-e5e31aec5664