#8 epub_tit_next crashes on navPoint without navLabel

[Stefan Brüns]

there is no check in the parser if a navPoint tocItem has a label:
https://sourceforge.net/p/ebook-tools/code/HEAD/tree/trunk/ebook-tools/src/libepub/opf.c#l401

When the label is accessed from epub_tit_next
https://sourceforge.net/p/ebook-tools/code/HEAD/tree/trunk/ebook-tools/src/libepub/epub.c#l473

label->compare is set even when label is a nullptr:
https://sourceforge.net/p/ebook-tools/code/HEAD/tree/trunk/ebook-tools/src/libepub/opf.c#l887