Fix a bug in the segment-to-segment linking part of sparse tracker.

This bug has the same causes than the previous one, and that affected
the frame-to-frame linking. It has of course the same fix.
I just wished I had discovered it before releasing v2.5.2.
Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>
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