[fix] Prevent crash when no page boxes (#5289)
[fix] Prevent crash when no page boxes (#5289)

Can occur with invalid page numbers, for example by changing the font size in a reflowable MuPDF document.

Discussion in <https://github.com/koreader/koreader/pull/5282#issuecomment-526842921>.
Hzj-jie/koreader,Markismus/koreader,koreader/koreader,pazos/koreader,mihailim/koreader,NiLuJe/koreader,poire-z/koreader,poire-z/koreader,Frenzie/koreader,mwoz123/koreader,koreader/koreader,Frenzie/koreader,NiLuJe/koreader