This avoids displaying duplicate content of the home page under different URLs with appended page numbers.
This change only affects the `<!--nextpage-->` pagination (`page` query variable) and not the regular multiple posts pagination (`paged` query variable).
The posts page does not support the `<!--nextpage-->` pagination, so requests for invalid page numbers should be redirected to the page permalink, applying the logic previously implemented for single posts or pages.
Follow-up to [34492], [47727].
Props jeremyfelt, sachit.tandukar, SergeyBiryukov.
Fixes#45337. See #40773, #28081, #11694.
git-svn-id: https://develop.svn.wordpress.org/trunk@47760 602fd350-edb4-49c9-b593-d223f7449a82
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