Minor code improvement to field existence check in redirect_fetch_rids_by_path

redirect_fetch_rids_by_path() checks that the 'status' column exists in the 'redirect' table before attempting to query on it. The Drupal function db_field_exists() queries the table every time it is called. If redirect_fetch_rids_by_path() is called several times per page (as seems to be the case when a node has several redirects to it), this leads to several unnecessary database queries. It should be safe to make checking for the column a static, as the field will either exist or not exist in a given page request.