Fix progress probe returning "done" when upload is still in progress.
Fix progress probe returning "done" when upload is still in progress.

This was due by reusing slabs from a terminated upload without reseting the
various parameters including the done parameter.

Tiger66639/nginx-upload-progress-module,gencer/nginx-upload-progress-module,masterzen/nginx-upload-progress-module,gencer/nginx-upload-progress-module,Tiger66639/nginx-upload-progress-module,masterzen/nginx-upload-progress-module,perusio/nginx-upload-progress-module,perusio/nginx-upload-progress-module