fix crash when opening TOC window in scroll mode of EPUB
fix crash when opening TOC window in scroll mode of EPUB

ashang/koreader,mihailim/koreader,chrox/koreader,apletnev/koreader,mwoz123/koreader,Hzj-jie/koreader,koreader/koreader,NiLuJe/koreader,frankyifei/koreader,Frenzie/koreader,robert00s/koreader,noname007/koreader,Markismus/koreader,pazos/koreader,Frenzie/koreader,chihyang/koreader,NickSavage/koreader,poire-z/koreader,koreader/koreader,NiLuJe/koreader,houqp/koreader,ashhher3/koreader,lgeek/koreader,poire-z/koreader