htlcswitch: check for signature owed in link

Previously the channel method FullySynced was used to decide whether to
send a new commit sig message. However, it could happen that FullySynced
was false, but that we didn't owe a commitment signature. Instead we
were waiting on the other party to send us a signature. If that
happened, we'd send out an empty commit sig. This commit modifies the
condition that triggers a new commit sig and fixes this deviation from
the spec.

htlcswitch/link.go

@@ -1103,7 +1103,7 @@ out:
 			// update in some time, check to see if we have any
 			// pending updates we need to commit due to our
 			// commitment chains being desynchronized.
-			if l.channel.FullySynced() {
+			if !l.channel.OweCommitment(true) {
 				continue
 			}
 
@@ -1808,7 +1808,7 @@ func (l *channelLink) handleUpstreamMsg(msg lnwire.Message) {
 		// If both commitment chains are fully synced from our PoV,
 		// then we don't need to reply with a signature as both sides
 		// already have a commitment with the latest accepted.
-		if l.channel.FullySynced() {
+		if !l.channel.OweCommitment(true) {
 			return
 		}