Followup to 51a6fb8c: Handle tags with zero-wikitext-tag-width

* Eliminates RT regression from it:Seconda_battaglia_dell%27Artois
  where a stray </p> was serialized back.

  Not sure why the stray </p> was generated in the HTML in the
  first place, but it shouldn't be auto-marked 'stx:html'

Change-Id: I7f64757df150482805a52b6a9c3f840f26b927a6