Fixed bug in block differences transfer code, that occurred if a file of size zero was updated via patching
Fixed bug in block differences transfer code, that occurred if a file of size zero was updated via patching

uroni/urbackup_backend,uroni/urbackup_backend,uroni/urbackup_backend,uroni/urbackup_backend,uroni/urbackup_backend,uroni/urbackup_backend,uroni/urbackup_backend