(Bug 53429) Fixed buggy fostered-node handling in DSR computation

* The existing DSR computation code did try to account for
  fostered nodes, but not all the way -- added a missing reset
  of dsr[1] after processing a fostered node.

* Tested on the following snippet by generating HTML, editing that
  HTML and selsering the edited HTML.
-----------------------
Paragraph.
{|
| abc
|-[[Foo]]
|}
-----------------------

  We need an update to our testing setup to enable this (since
  this requires an old html, old wt, and an edited html which
  cannot currently be represented in parserTests.txt).

* Tested on fr:Villes_aux_États-Unis with the same process as
  on the wikitext snippet above.

Change-Id: Ied3fda48626312e58860e1b49e04c6625243ed7b