fix: fix a misplaced unlock (ethersphere#2124)

Author: ralph-pichler

pkg/accounting/accounting.go

@@ -197,13 +197,12 @@ func (a *Accounting) Reserve(ctx context.Context, peer swarm.Address, price uint
 	accountingPeer := a.getAccountingPeer(peer)
 
 	accountingPeer.lock.Lock()
+	defer accountingPeer.lock.Unlock()
 
 	if !accountingPeer.connected {
 		return fmt.Errorf("connection not initialized yet")
 	}
 
-	defer accountingPeer.lock.Unlock()
-
 	a.metrics.AccountingReserveCount.Inc()
 	bigPrice := new(big.Int).SetUint64(price)