IsValidOp: throw proper error on nested shells (#608)

Thanks geomworx

git-svn-id: http://svn.osgeo.org/geos/branches/3.3@3756 5242fede-7e19-0410-aef8-94bd7d2200fb
Prepare codebase for next 3.3 release (3.3.8)

git-svn-id: http://svn.osgeo.org/geos/branches/3.3@3757 5242fede-7e19-0410-aef8-94bd7d2200fb