Bugfix: #27 #33 #45 Avoid wp_authenticate's errors

[BrianHenryIE]

Sometimes, with an incorrect username and password (or using email address rather than username, which made wp_authenticate_username_password error) when $user = wp_authenticate( $username, $password ); errored, it would call $wp_rewrite->get_page_permastruct(); before $wp_rewrite was initialized, returning a 500 server error. In my case, this would happen when I enabled the SkyVerge WooCommerce Print Invoices/Packing Lists plugin.