Added check for $screen in filter_manage_posts_columns.

This will resolve the following PHP Notice: Notice: Trying to get property of non-object in htdocs/wp-content/plugins/edit-flow/modules/editorial-metadata/editorial-metadata.php on line 634

Ref: http://wordpress.org/support/topic/notice-trying-to-get-property-of-non-object-in-fn-filter_manage_posts_columns?replies=4#post-4703836