Transaction.addSignedInput: catch API usage error.

core/src/main/java/com/google/bitcoin/core/Transaction.java

@@ -36,6 +36,7 @@ import java.text.SimpleDateFormat;
 import java.util.*;
 
 import static com.google.bitcoin.core.Utils.*;
+import static com.google.common.base.Preconditions.checkState;
 
 /**
  * <p>A transaction represents the movement of coins from some addresses to some other addresses. It can also represent
@@ -744,6 +745,8 @@ public class Transaction extends ChildMessage implements Serializable {
      */
     public TransactionInput addSignedInput(TransactionOutPoint prevOut, Script scriptPubKey, ECKey sigKey,
                                            SigHash sigHash, boolean anyoneCanPay) throws ScriptException {
+        // Verify the API user didn't try to do operations out of order.
+        checkState(!outputs.isEmpty(), "Attempting to sign tx without outputs.");
         TransactionInput input = new TransactionInput(params, this, new byte[]{}, prevOut);
         addInput(input);
         Sha256Hash hash = hashForSignature(inputs.size() - 1, scriptPubKey, sigHash, anyoneCanPay);