cnct: do not create an action for incoming dust htlcs

This commit fixes the 'unable to find incoming resolution' error that
occured when trying to resolve incoming htlcs below the dust limit that
are not actually present on the commitment tx.

contractcourt/channel_arbitrator.go

@@ -1316,6 +1316,15 @@ func (c *ChannelArbitrator) checkCommitChainActions(height uint32,
 	// either learn of it eventually from the outgoing HTLC, or the sender
 	// will timeout the HTLC.
 	for _, htlc := range htlcs.incomingHTLCs {
+		// If the HTLC is dust, there is no action to be taken.
+		if htlc.OutputIndex < 0 {
+			log.Debugf("ChannelArbitrator(%v): no resolution "+
+				"needed for incoming dust htlc=%x",
+				c.cfg.ChanPoint, htlc.RHash[:])
+
+			continue
+		}
+
 		log.Tracef("ChannelArbitrator(%v): watching chain to decide "+
 			"action for incoming htlc=%x", c.cfg.ChanPoint,
 			htlc.RHash[:])

contractcourt/channel_arbitrator_test.go

@@ -527,8 +527,15 @@ func TestChannelArbitratorLocalForceClosePendingHtlc(t *testing.T) {
 		OutputIndex: -1,
 	}
 
+	incomingDustHtlc := channeldb.HTLC{
+		Incoming:    true,
+		Amt:         105,
+		HtlcIndex:   101,
+		OutputIndex: -1,
+	}
+
 	htlcSet := []channeldb.HTLC{
-		htlc, outgoingDustHtlc,
+		htlc, outgoingDustHtlc, incomingDustHtlc,
 	}
 
 	htlcUpdates <- &ContractUpdate{