Disk usage issue

We have a disk space available check in the save sketch api function, but it has happened multiple times that we found boxes with 0 kb available. If this happens boxes boxes become unresponsive, sometimes you can't even ssh into them. (You can still go into failsave mode). 
Our current check: 
https://github.com/Doodle3D/doodle3d-firmware/blob/master/src/rest/api/api_sketch.lua#L121-L127
