Bookmarks: avoid possible crash
Bookmarks: avoid possible crash

(Not sure how and with what this check works and if
we still get such kind of items, but avoid a crash
when item2.pos0 exists but item1.pos0 does not.)

NiLuJe/koreader,Frenzie/koreader,pazos/koreader,NiLuJe/koreader,Markismus/koreader,Frenzie/koreader,poire-z/koreader,koreader/koreader,Hzj-jie/koreader,koreader/koreader,mwoz123/koreader,poire-z/koreader