withdraw: fix incorrect error when we have an empty wallet.

This also highlights the danger of searching the logs: that error
appeared previously in the logs, so we didn't notice that the actual
withdraw call gave a different error.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

common/wallet_tx.c

@@ -14,7 +14,7 @@ void wtx_init(struct command *cmd, struct wallet_tx * wtx)
 
 static bool check_amount(const struct wallet_tx *tx, u64 amount)
 {
-	if (!tx->utxos) {
+	if (tal_count(tx->utxos) == 0) {
 		command_fail(tx->cmd, FUND_CANNOT_AFFORD,
 			     "Cannot afford transaction");
 		return false;