Fix missing edge from toTopoGeom return (#1968)

git-svn-id: http://svn.osgeo.org/postgis/branches/2.0@11153 b70326c6-7e19-0410-871a-916f4a2858ee
Do not assume TopoGeometry definition only changes by our own calls

Fixes the original case in #1968

git-svn-id: http://svn.osgeo.org/postgis/branches/2.0@11154 b70326c6-7e19-0410-871a-916f4a2858ee