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:

geogram/src/lib/geogram/voronoi/convex_cell.cpp

Line 860 in 982abdb

vbw_assert(nb2 == nb);

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