Zero initialize values in vtkGradientFilter

This fixes a "Conditional jump or move depends on uninitialised
value(s)" error from valgrind by zero initializing the values at
the top of ComputeGradientsSG().

Change-Id: Idfc249ed797c798a9bd79ece2883ca48d9735afb
Merge topic 'zero-initialize-in-gradient-filter' into master

d33f2b8c Zero initialize values in vtkGradientFilter