lnwallet/btcwallet: check output is under our control in FetchInputInfo

In this commit, we add an additional check to btcwallet's FetchInputInfo
method to ensure the output is actually under control of the wallet.
Previously, the wallet would assume the output was under its control if
the txid of the output was found within the wallet. This is not a safe
assumption to make however, because if we happened to be the sender of
this transaction, it would be found within the wallet but it's not
actually under our control. To fix this, we explicitly check that there
exists an address in our wallet for this output.

lnwallet/btcwallet/signer.go

@@ -44,7 +44,14 @@ func (b *BtcWallet) FetchInputInfo(prevOut *wire.OutPoint) (*wire.TxOut, error)
 		return nil, lnwallet.ErrNotMine
 	}
 
+	// With the output retrieved, we'll make an additional check to ensure
+	// we actually have control of this output. We do this because the check
+	// above only guarantees that the transaction is somehow relevant to us,
+	// like in the event of us being the sender of the transaction.
 	output = txDetail.TxRecord.MsgTx.TxOut[prevOut.Index]
+	if _, err := b.fetchOutputAddr(output.PkScript); err != nil {
+		return nil, err
+	}
 
 	b.cacheMtx.Lock()
 	b.utxoCache[*prevOut] = output
@@ -72,7 +79,7 @@ func (b *BtcWallet) fetchOutputAddr(script []byte) (waddrmgr.ManagedAddress, err
 		}
 	}
 
-	return nil, errors.Errorf("address not found")
+	return nil, lnwallet.ErrNotMine
 }
 
 // fetchPrivKey attempts to retrieve the raw private key corresponding to the
@@ -196,7 +203,7 @@ func (b *BtcWallet) ComputeInputScript(tx *wire.MsgTx,
 	outputScript := signDesc.Output.PkScript
 	walletAddr, err := b.fetchOutputAddr(outputScript)
 	if err != nil {
-		return nil, nil
+		return nil, err
 	}
 
 	pka := walletAddr.(waddrmgr.ManagedPubKeyAddress)