Make num_links=0 in pagination library possible to configure

In pagination library, num_links = 0 wasn't supported so far.
So I modified system/libraries/Pagination.php and made num_links = 0 configuration available.
Similarly, revert the changes *inside* this codeblock and change the condition to:

    if ($this->last_link && ($this->cur_page + $this->num_links + ! $this->num_links) < $num_pages)