Planner Crashes if Repeat is less than 5 vertices long

[a-krawciw]

The condition fails because of this line. https://github.com/utiasASRL/vtr3/blob/d74564fff70c21f517391c9b785cb0287d7314e6/main/src/vtr_path_planning/src/cbit/generate_pq.cpp#L108 The length of the list is a single point. Spline interpolation fails. Add a check to use a smaller reference for very short paths.