avoid doing zero-size refreshes
avoid doing zero-size refreshes

Those refresh requests will crash (at least) Kobos' framebuffer
driver and create the image of a locked-up device.
Beware: device is not actually locked up and does in fact react
to touch screen events!

Hzj-jie/koreader-base,koreader/koreader-base,Frenzie/koreader-base,houqp/koreader-base,NiLuJe/koreader-base,koreader/koreader-base,koreader/koreader-base,apletnev/koreader-base,houqp/koreader-base,frankyifei/koreader-base,apletnev/koreader-base,Frenzie/koreader-base,apletnev/koreader-base,Hzj-jie/koreader-base,Hzj-jie/koreader-base,NiLuJe/koreader-base,NiLuJe/koreader-base,frankyifei/koreader-base,houqp/koreader-base,Frenzie/koreader-base,frankyifei/koreader-base,frankyifei/koreader-base,apletnev/koreader-base,Frenzie/koreader-base,NiLuJe/koreader-base,Hzj-jie/koreader-base,houqp/koreader-base,koreader/koreader-base