tapchannel: fail early if we'd send 0 assets

Fixes #1006.

In this commit we make sure that we'll never end up attempting to send a
zero asset amount HTLC, which would then fail further down the line.
A zero amount can happen if the resolution of asset unit to satoshis is
insufficient and one unit costs more than the satoshi amount that should
be represented in the HTLC.
The main mitigation for this is on the issuer side that they make sure
the decimal display value is high enough and one asset unit represents a
fraction of a cent (and is therefore close to being equal to one
satoshi).

Author: guggero

tapchannel/aux_traffic_shaper.go

@@ -256,6 +256,17 @@ func (s *AuxTrafficShaper) ProduceHtlcExtraData(totalAmount lnwire.MilliSatoshi,
 		return 0, nil, fmt.Errorf("quote has no asset ID")
 	}
 
+	// If the number of asset units to send is zero due to integer division
+	// and insufficient asset unit precision vs. satoshis, we cannot send
+	// this payment. This should only happen if the amount to pay is very
+	// small (small satoshi or sub satoshi total value) or the price oracle
+	// has given a very high price for the asset.
+	if numAssetUnits == 0 {
+		return 0, nil, fmt.Errorf("asset unit price (%d mSat per "+
+			"asset unit) too high to represent HTLC value of %v",
+			mSatPerAssetUnit, totalAmount)
+	}
+
 	log.Debugf("Producing HTLC extra data for RFQ ID %x (SCID %d): "+
 		"asset ID %x, asset amount %d", rfqID[:], rfqID.Scid(),
 		quote.Request.AssetID[:], numAssetUnits)