wallet: fix nil txid in publishTransaction

A nil txid could've been returned from publishTransaction even if it was
successful. This was due to the underlying SendRawTransaction call
"failing", e.g., when the transaction being broadcast has already
confirmed, but publishTranasction interpreting such failure as a
success.

wallet/wallet.go

@@ -3385,7 +3385,7 @@ func (w *Wallet) publishTransaction(tx *wire.MsgTx) (*chainhash.Hash, error) {
 		return nil, err
 	}
 
-	txid, err := chainClient.SendRawTransaction(tx, false)
+	_, err = chainClient.SendRawTransaction(tx, false)
 
 	// Determine if this was an RPC error thrown due to the transaction
 	// already confirming.
@@ -3394,9 +3394,10 @@ func (w *Wallet) publishTransaction(tx *wire.MsgTx) (*chainhash.Hash, error) {
 		rpcTxConfirmed = rpcErr.Code == btcjson.ErrRPCTxAlreadyInChain
 	}
 
+	txid := tx.TxHash()
 	switch {
 	case err == nil:
-		return txid, nil
+		return &txid, nil
 
 	// Since we have different backends that can be used with the wallet,
 	// we'll need to check specific errors for each one.
@@ -3415,7 +3416,7 @@ func (w *Wallet) publishTransaction(tx *wire.MsgTx) (*chainhash.Hash, error) {
 	case strings.Contains(
 		strings.ToLower(err.Error()), "txn-already-in-mempool",
 	):
-		return txid, nil
+		return &txid, nil
 
 	// If the transaction has already confirmed, we can safely remove it
 	// from the unconfirmed store as it should already exist within the
@@ -3450,7 +3451,7 @@ func (w *Wallet) publishTransaction(tx *wire.MsgTx) (*chainhash.Hash, error) {
 				"from unconfirmed store: %v", tx.TxHash(), dbErr)
 		}
 
-		return txid, nil
+		return &txid, nil
 
 	// If the transaction was rejected for whatever other reason, then we'll
 	// remove it from the transaction store, as otherwise, we'll attempt to