[#326] settlement/audit: Prevent negative amount arguments of Exchanger

Do not pass zero transfers from the calculation table to Exchanger. Revert
transfers with negative amount since Exchanger interface requires positive
amounts of funds.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

pkg/innerring/processors/settlement/audit/calculate.go

@@ -82,6 +82,17 @@ func (c *Calculator) Calculate(p *CalculatePrm) {
 	log.Debug("processing transfers")
 
 	table.iterate(func(tx *transferTx) {
+		sign := tx.amount.Sign()
+		if sign == 0 {
+			log.Debug("ignore zero transfer")
+			return
+		}
+
+		if sign < 0 {
+			tx.from, tx.to = tx.to, tx.from
+			tx.amount.Neg(tx.amount)
+		}
+
 		c.prm.Exchanger.Transfer(tx.from, tx.to, tx.amount)
 	})
 }