initialize from_start_to_end with None

[lbenedix]

No description provided.

[lbenedix]

This makes the function explode

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<gpx>
  <trk>
    <trkseg>
      <trkpt lon="8.354584" lat="51.481570">
      </trkpt>
      <trkpt lon="8.354575" lat="51.481573">
      </trkpt>
      <trkpt lon="8.354575" lat="51.481573">
      </trkpt>
    </trkseg>
  </trk>
</gpx>

  File "/usr/lib/python3/dist-packages/gpxpy/gpx.py", line 1112, in <listcomp>
    return [(distance / from_start_to_end) if from_start_to_end else 0
NameError: free variable 'from_start_to_end' referenced before assignment in enclosing scope