if *only* data chunks are registered (high chance during incremental
backup), then chunk_count might be one lower then upload_stat.count
because of the zero chunk being unconditionally uploaded but not used.
Thus when subtracting the two, an overflow would occur.
In general, don't let the client make the server panic, instead just set
duplicates to 0.
Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>
8268c9d161