Fixed null dereference

If the big for loop is never entered,
then slice1scalars would be null and dereferenced

Change-Id: I4c8300c0b082c9f1a8349659f83297f81efa9894

Imaging/Hybrid/vtkSliceCubes.cxx

@@ -138,7 +138,7 @@ int vtkSliceCubesContour(T *slice, S *scalars, int imageRange[2], int dims[3],
                          double xmin[3], double xmax[3], FILE *outFP,
                          vtkVolumeReader *reader, unsigned char debug)
 {
-  S *slice0scalars=NULL, *slice1scalars;
+  S *slice0scalars=NULL, *slice1scalars=NULL;
   S *slice2scalars, *slice3scalars;
   T *slice0, *slice1, *slice2, *slice3;
   vtkImageData *sp;
@@ -166,7 +166,6 @@ int vtkSliceCubesContour(T *slice, S *scalars, int imageRange[2], int dims[3],
     doubleScalars->Allocate(sliceSize);
     }
 
-  slice1scalars = NULL;
   slice2scalars = scalars;
   slice2scalars->Register(NULL);
 
@@ -387,7 +386,10 @@ int vtkSliceCubesContour(T *slice, S *scalars, int imageRange[2], int dims[3],
     {
     slice3scalars->Delete();
     }
-  slice1scalars->Delete();
+  if (slice1scalars)
+    {
+    slice1scalars->Delete();
+    }
   slice2scalars->Delete();
 
   return numTriangles;