lnwallet/btcwallet: fix bug in implementation of GetUxo

This commit fixes a bug within the btcwallet implementation of the
BlockChainIO interface. The exact nature of the bug was a rounding
error that would only manifest if the value of the UTXO was below 1
BTC.

The tests within this package currently test channels with mostly whole
values of BTC, as a result the bug went unnoticed until now.

The fix itself is trivial: convert to an int64 AFTER performing the
multiplication to convert to satoshis from Bitcoin.

lnwallet/btcwallet/blockchain.go

@@ -32,7 +32,7 @@ func (b *BtcWallet) GetUtxo(txid *wire.ShaHash, index uint32) (*wire.TxOut, erro
 	return &wire.TxOut{
 		// Sadly, gettxout returns the output value in BTC
 		// instead of satoshis.
-		Value:    int64(txout.Value) * 1e8,
+		Value:    int64(txout.Value * 1e8),
 		PkScript: pkScript,
 	}, nil
 }