fixed "direction" issue for zero-length lines

git-svn-id: https://pycam.svn.sourceforge.net/svnroot/pycam/trunk@858 bbaffbd6-741e-11dd-a85d-61de82d9cad9

src/pycam/Gui/OpenGLTools.py

@@ -651,6 +651,9 @@ def draw_direction_cone(p1, p2):
     distance = p2.sub(p1)
     length = distance.norm
     direction = distance.normalized()
+    if direction is None:
+        # zero-length line
+        return
     cone_radius = length / 30
     cone_length = length / 10
     # move the cone to the middle of the line