accounting/tally: if delta < 0, delta = 0

if redis crashed in the middle of tally we could have a situation
where we erroneously subtract from a project total. Currently,
`latest` should never be less than `initial`

Change-Id: Ibb5ab724ac0ad4d684f7954fad7a9e061104b7df

satellite/accounting/tally/tally.go

@@ -132,6 +132,9 @@ func (service *Service) Tally(ctx context.Context) (err error) {
 
 		for projectID, latest := range latestLiveTotals {
 			delta := latest - initialLiveTotals[projectID]
+			if delta < 0 {
+				delta = 0
+			}
 			err = service.liveAccounting.AddProjectStorageUsage(ctx, projectID, -latest+tallyProjectTotals[projectID]+(delta/2))
 			if err != nil {
 				return Error.Wrap(err)