[vtkusers] Unconnected filter(s) causing seg faults

[N Smethurst]

I just went through the Graphics directory and found about 4/5 without 
checks. I put the list at 

kvisualiser.free.fr/vtk/null_input_check 

if you're interested in looking at it. There are about 100 without checks.. I 
haven't checked any other directories.

How should the checks be implemented? I noticed that in the methods that do 
check, some do this

  if (input == NULL)
    {
    vtkErrorMacro(<<"Input is NULL");
    return;
    }

whilst others just do a 

  if (input == NULL)
    {
    return;
    }