Jetpack: Infinite Scroll: prevent ''Undefined index'' notice that can cause IS to fail when user has `WP_DEBUG` set to `true`.

If a user has IS enabled and `WP_DEBUG` is `true`, an ''Undefined index'' notice could result when determining the sort order for new posts. As a result, the IS Ajax endpoint doesn't return valid JSON, causing IS to fail when parsing the returned value from the Ajax endpoint.


git-svn-id: https://plugins.svn.wordpress.org/jetpack/trunk@815022 b8457f37-d9ea-0310-8a92-e5e31aec5664