don't crash on bad input data


git-svn-id: https://geuz.org/svn/gmsh/trunk@19745 59c8cabf-10d6-4a28-b43f-3936b85956c4
pp


git-svn-id: https://geuz.org/svn/gmsh/trunk@19746 59c8cabf-10d6-4a28-b43f-3936b85956c4