Use asserts in dbl_list

codegen/dbl_list/snippets/list_pop_back

@@ -1,6 +1,8 @@
+  assert(list != NULL);
+
   {NodeType} *output = NULL;
 
-  if (!list || list->node_count == 0) {{
+  if (list->node_count == 0) {{
     goto RETURN_{Tupper}_LIST_POP_BACK;
   }}