example: Fix insufficient validation of the head of cleanup node list

In the previous commit(8164e1ff), the NULL check for head_pre is removed and the NULL check for *head_pre is added. But the new check doesn't really work, because *head_pre can be assigned to new_node->next, whether it's NULL or not. In addition, *head_pre is not checked before the dereference below.

The cleanup_push() is used to add new_data to a cleanup node list. The *head_pre being NULL means that the list does not exist, which makes cleanup_push meaningless. Thus, the *head_pre cannot be NULL, when cleanup_push() is called.