callgraph-view: compare nodes by using data field

We could have multiple StackNodes that point at the same data. However
we might not have pointer equality. This uses the data pointer that nodes
point at to determine equality.

lib/sp-callgraph-view.c

@@ -526,7 +526,7 @@ sp_callgraph_view_set_node (SpCallgraphView *self,
                               COLUMN_POINTER, &item,
                               -1);
 
-          if (item == node)
+          if (item != NULL && item->data == node->data)
             {
               GtkTreeSelection *selection;