Fix LuaCheck error
Fix LuaCheck error

"frontend/ui/menusorter.lua:110:13: value assigned to variable 'sub_menu_content' is unused"

Kind of the point, I'm assigning it nil after all. This way it probably shouldn't complain.

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