Correct rewrite check

[meloniq]

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