better error


git-svn-id: https://geuz.org/svn/gmsh/trunk@18411 59c8cabf-10d6-4a28-b43f-3936b85956c4
fix logic to avoid negative array index


git-svn-id: https://geuz.org/svn/gmsh/trunk@18412 59c8cabf-10d6-4a28-b43f-3936b85956c4