Check for non-null DSR in handleLinkNeighbors before updating it.

* en:Advaita Vedanta was crashing with undefined dsr -- fixed
  with this patch.

* Minor tweak in DOMTraverser.callHandlers

* No change in parser tests.

Change-Id: Ie562be34b8bed63a65eadece2c24954130b1221a
Merge "Check for non-null DSR in handleLinkNeighbors before updating it."