Statistics: Guard against ReaderView's state.page being nil
Statistics: Guard against ReaderView's state.page being nil

For... whatever reason? I can't really see that happening, but, oh,
well.

Screensaver already guards against it, so, who knows.

Fix #8201

poire-z/koreader,koreader/koreader,Frenzie/koreader,koreader/koreader,NiLuJe/koreader,poire-z/koreader,Frenzie/koreader,NiLuJe/koreader