Fix #34972: Blender crashes when Force Field-> Curve Guide is created

Made it so where_on_path works correct when Curve does
have editNurbs but no Curve->nurbs.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@56060 954f8c5b-7b00-dc11-b283-0030488c597c