Use asserts in dbl_list

codegen/dbl_list/snippets/list_push_back

@@ -1,6 +1,4 @@
-  if (!list || !node || !(node->item)) {{
-    return;
-  }}
+  assert(list != NULL && node != NULL && (node->item) != NULL);
 
   {ListType} node_list = {Tlower}_node_to_list(node);