fix OTA server broken on checking stable release
fix OTA server broken on checking stable release

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