fix dictionary lookup crash in scroll mode
fix dictionary lookup crash in scroll mode

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