Fix for sitemap issue which caused all pages of a split sitemap to be the same

Commit https://github.com/Yoast/wordpress-seo/commit/f01b2a644bfc8f4b055223fcdccd6f3850551afe reverted a change in how the redirect variables where picked up, but was incomplete.
The page number of the sitemap was no longer being set which caused all sitemaps to be the same, i.e. show the first 1000 (or $max_entries) urls.

This pull request fixed that.

As reported by @casacochecurro in issue #486 and @awhig in #484