Prevent undefined behaviour when drawing axis

Drawing XYZ axis with a negative size makes no sense
Worse, drawing axis with a 0 size may cause errors :

```
Warning: In /build/buildd/vtk-5.8.0/Graphics/vtkTubeFilter.cxx, line 362
vtkTubeFilter (0x1fbaae0): Coincident points!

Warning: In /build/buildd/vtk-5.8.0/Graphics/vtkTubeFilter.cxx, line 236
vtkTubeFilter (0x1fbaae0): Could not generate points!

Warning: In /build/buildd/vtk-5.8.0/Graphics/vtkTubeFilter.cxx, line 362
vtkTubeFilter (0x1fbaae0): Coincident points!

Warning: In /build/buildd/vtk-5.8.0/Graphics/vtkTubeFilter.cxx, line 236
vtkTubeFilter (0x1fbaae0): Could not generate points!

Warning: In /build/buildd/vtk-5.8.0/Graphics/vtkTubeFilter.cxx, line 362
vtkTubeFilter (0x1fbaae0): Coincident points!

Warning: In /build/buildd/vtk-5.8.0/Graphics/vtkTubeFilter.cxx, line 236
vtkTubeFilter (0x1fbaae0): Could not generate points!
```

I solved the problem by transforming bad values into the default value (size = 1.0) without throwing any error/warning