fix out-of-bound bug in filemanager
fix out-of-bound bug in filemanager

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