fix minor issue with closed ways that are not connected to other roads
simplify code: use Coord.decHighwayCount() instead of creating a copy 
of Coord. This has a small (positive) side effect if the node
is also used in other ways.

git-svn-id: http://svn.mkgmap.org.uk/mkgmap/trunk@3331 25d90789-57f7-4ee0-8453-03a3dfeeeb22