fix error in case when process old statistics files
fix error in case when process old statistics files

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