This changes the SetNumberOfInformationObjects() method in the
vtkInformationVector class to only set the number of information
objects if it is different from the current number of objects.

This fixes an issue in which multiple threads calling the
vtkExecutive::GetOutputInfomation() method would result in
SetNumberOfInformationObjects() being called simultaneously.

Change-Id: Ib39818d163ffa514308c72489cfaa2dcac9cd7fa