Base vertex snap tolerance in lwline_split_by_point on line length

Fixes robustness issue in splitting line with own vertex (#2173)
Also fixes one case of topology building (#2172)

git-svn-id: http://svn.osgeo.org/postgis/branches/2.0@10972 b70326c6-7e19-0410-871a-916f4a2858ee