Infinite loop or crash with ConvexCell

[BrunoLevy]

To reproduce the bug

• start geogram_demo_Delaunay3d
• set N to 9 points
• Reset until you get the symmetric configuration with an octahedral cell in the middle (thank you Laurent Astart for having reported this one)

In debug mode, assertion fail here: https://github.com/BrunoLevy/geogram/blob/982abdb6429fb48ae97bac574b633f5f0fa363c6/src/lib/geogram/voronoi/convex_cell.cpp#L860

In the debugger:

  nb  = 8
  nb2 = 4

(mismatch between number of vertices on border of conflict zone obtained during two consecutive traversals)

Happens also with N=8