Check if chapter xhtml is integer

[maggie44]

I think the distinction for this line is whether the response is an integer, rather than whether it is above 0?

I came across an ePub that had -1 as the returned string and caused and error as it was supposed to be processed as this.chapter but was passed on to epubcfi (https://contentserver.adobe.com/store/books/GeographyofBliss_oneChapter.epub).