16130: Fix inconsistency in vtkImageSliceCollection.

The Bottom member of vtkCollection was not updated if the added slice
replaced the previous bottom slice of the collection.  This caused an
inconsistency in the data structure which could lead to a crash in
subsequent operations.  I have rewritten the insertion code to reduce
its complexity.

Rendering/Image/vtkImageSliceCollection.cxx

@@ -41,20 +41,10 @@ vtkImageSliceCollection::~vtkImageSliceCollection()
 void vtkImageSliceCollection::AddItem(vtkImageSlice *a)
 {
   vtkCollectionElement* elem = new vtkCollectionElement;
+  elem->Item = a;
 
-  // Check if the top item is NULL
-  if (this->Top == NULL)
-    {
-    this->Top = elem;
-    elem->Item = a;
-    elem->Next = NULL;
-    this->Bottom = elem;
-    this->NumberOfItems++;
-    a->Register(this);
-    return;
-    }
-
-  // Insert according to the layer number
+  // Find insertion location according to the layer number
+  vtkCollectionElement *prevElem = 0;
   int layerNumber = a->GetProperty()->GetLayerNumber();
   for (vtkCollectionElement *indexElem = this->Top;
        indexElem != 0;
@@ -63,23 +53,29 @@ void vtkImageSliceCollection::AddItem(vtkImageSlice *a)
     vtkImageSlice* tempImage = static_cast<vtkImageSlice*>(indexElem->Item);
     if (layerNumber < tempImage->GetProperty()->GetLayerNumber())
       {
-      // The indexElem item's layer number is larger, so swap
-      // the new item and the indexElem item.
-      elem->Item = indexElem->Item;
-      elem->Next = indexElem->Next;
-      indexElem->Item = a;
-      indexElem->Next = elem;
-      this->NumberOfItems++;
-      a->Register(this);
-      return;
+      break;
       }
+    prevElem = indexElem;
+    }
+
+  // Insert the new element into the linked list
+  if (prevElem == 0)
+    {
+    elem->Next = this->Top;
+    this->Top = elem;
+    }
+  else
+    {
+    elem->Next = prevElem->Next;
+    prevElem->Next = elem;
+    }
+
+  // Check if this is the new bottom
+  if (elem->Next == 0)
+    {
+    this->Bottom = elem;
     }
 
-  //End of list found before a larger layer number
-  elem->Item = a;
-  elem->Next = NULL;
-  this->Bottom->Next = elem;
-  this->Bottom = elem;
   this->NumberOfItems++;
   a->Register(this);
 }