Fix OffsetCurve op in presence of duplicated vertices (#602)

git-svn-id: http://svn.osgeo.org/geos/trunk@3845 5242fede-7e19-0410-aef8-94bd7d2200fb
Make GEOSOffsetCurve survive single-point input (with an exception)

git-svn-id: http://svn.osgeo.org/geos/trunk@3846 5242fede-7e19-0410-aef8-94bd7d2200fb