[vtkusers] bug report + fix

[alext at win.tue.nl]

  Hi all,

  I've already reported a bug in vtkAppendFilter's implementation.
  The bug causes the filter to crash if one appends and deletes
  inputs in a random order. The bug is located in the
  vtkAppendFilter::Execute() method (lines 333,334 in the nightly
  release). These lines should be moved _inside_ the for loop
  ending on line 332 - i.e., they should be executed ONLY when
  the method processes a non-NULL input.

  I've already reported this bug to the vtk mailing list some time ago.
  Since it is not something trivial to spot, it would be nice if it
  were corrected in the upcoming nightly releases.

  Alex

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  Alexandru C. Telea   	      | Technische Universiteit Eindhoven
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  E-mail   : alext at win.tue.nl | Dept. of Mathematics and 
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