knife sort_by_frac_along was re-calculating the reference factor for every test, change to only calculate once and use line_point_factor_v3().
also add zero division check for line_point_factor_v3() since the 2d version already checked for this.


git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@56486 954f8c5b-7b00-dc11-b283-0030488c597c