[Gmsh] [seg. fault] points with negative geometric entity cause trouble when visualising a vector field by "displacement"

[mianzhi]

Hello developers,
The following is a summary of this bug:

platform:

    Linux 3.0.0-13-generic #22-Ubuntu SMP Wed Nov 2 13:27:26 UTC 2011
    x86_64 x86_64 x86_64 GNU/Linux


gmsh version:

    Version : 2.5.1
    GUI toolkit : FLTK 1.1.10
    License : GNU General Public License
    Build OS : Linux
    Build options : Ann Bamg Bfgs Blas Chaco DIntegration Dlopen FlTree
    Fltk Gmm Have64BitSizeT Jpeg Lapack MathEx Med Mesh Mmg3d Mpeg
    NativeFileChooser Netgen OpenCascade OpenGL Parser Plugins Png Post
    Salome Solver Zlib
    Build date : 20110911
    Build host : crested
    Packager : buildd
    Web site : http://www.geuz.org/gmsh/
    Mailing list : gmsh at geuz.org

how to trigger this seg. fault:

  * open the attached reduced .msh file by gmsh
  * tools/options/view[0]/aspect/vector_display : displacement

Thank you very much for the awesome software.
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