Correct rewrite check

Corrects rewrite check, might not be ```false```, but do not apply when the permalinks are disabled, also fixes undefined index notices

WP core checks:

 - post: https://core.trac.wordpress.org/browser/tags/3.8.1/src/wp-includes/post.php#L1279

 - taxonomy: https://core.trac.wordpress.org/browser/tags/3.8.1/src/wp-includes/taxonomy.php#L359