chanbackup: handle Single creation for zero-conf channels

The SCB function NewSingle is now zero-conf aware. Since the confirmed
short channel id may be unknown, it may use the broadcast height.

chanbackup/single.go

@@ -180,6 +180,24 @@ func NewSingle(channel *channeldb.OpenChannel,
 		chanID.BlockHeight = channel.BroadcastHeight()
 	}
 
+	// If this is a zero-conf channel, we'll need to have separate logic
+	// depending on whether it's confirmed or not. This is because the
+	// ShortChanID is an alias.
+	if channel.IsZeroConf() {
+		// If the channel is confirmed, we'll use the confirmed SCID.
+		if channel.ZeroConfConfirmed() {
+			chanID = channel.ZeroConfRealScid()
+		} else {
+			// Else if the zero-conf channel is unconfirmed, we'll
+			// need to use the broadcast height and zero out the
+			// TxIndex and TxPosition fields. This is so
+			// openChannelShell works properly.
+			chanID.BlockHeight = channel.BroadcastHeight()
+			chanID.TxIndex = 0
+			chanID.TxPosition = 0
+		}
+	}
+
 	single := Single{
 		IsInitiator:      channel.IsInitiator,
 		ChainHash:        channel.ChainHash,