`autop()` can sometimes get confused and not clean up stray `<p>` or `</p>` tags inside block level elements, which produces sub-optimal HTML. While browsers can generally handle it, there's no need to make things harder for them if we don't have to.
Props pento, ayubi, pbearne, jond, azaozz, 1994rstefan, dionysous, MikeHansenMe, jorbin, miqrogroove, niallkennedy.
Fixes#27350.
git-svn-id: https://develop.svn.wordpress.org/trunk@45585 602fd350-edb4-49c9-b593-d223f7449a82
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