This avoids displaying duplicate content of the same post under different URLs and ensures the canonical URL is correct.
Previously, requests for invalid page numbers were only redirected to the post permalink if the post was actually paginated using the `<!--nextpage-->` marker.
Follow-up to [34492].
Props jeremyfelt, prografika, sachit.tandukar, subrataemfluence, hronak, ekatherine, henry.wright, chesio, dd32, SergeyBiryukov.
Fixes#40773. See #45337, #28081, #11694.
git-svn-id: https://develop.svn.wordpress.org/trunk@47727 602fd350-edb4-49c9-b593-d223f7449a82
af596bcc5b