travis run: https://travis-ci.org/bitcoin/bitcoin/jobs/406813397

```
rpc_psbt.py failed, Duration: 4 s
stdout:
2018-07-22T13:28:57.765000Z TestFramework (INFO): Initializing test directory /tmp/bitcoin_test_runner_20180722_132530/rpc_psbt_49
2018-07-22T13:29:00.531000Z TestFramework (ERROR): Assertion failed
Traceback (most recent call last):
  File "/home/travis/build/bitcoin/bitcoin/build/bitcoin-x86_64-unknown-linux-gnu/test/functional/test_framework/test_framework.py", line 160, in main
    self.run_test()
  File "/home/travis/build/bitcoin/bitcoin/build/bitcoin-x86_64-unknown-linux-gnu/test/functional/rpc_psbt.py", line 101, in run_test
    assert_raises_rpc_error(-22, "TX decode failed", self.nodes[0].converttopsbt, signedtx['hex'])
  File "/home/travis/build/bitcoin/bitcoin/build/bitcoin-x86_64-unknown-linux-gnu/test/functional/test_framework/util.py", line 105, in assert_raises_rpc_error
    assert try_rpc(code, message, fun, *args, **kwds), "No exception raised"
AssertionError: No exception raised
for (const CMutableTransaction& txv : txVariants) {
            if (txv.vin.size() > i) {
                sigdata.MergeSignatureData(DataFromTransaction(txv, i, coin.out));
            }
        }
        ProduceSignature(DUMMY_SIGNING_PROVIDER, MutableTransactionSignatureCreator(&mergedTx, i, coin.out.nValue, 1), coin.out.scriptPubKey, sigdata);
        UpdateInput(txin, sigdata);
    }
    return EncodeHexTx(CTransaction(mergedTx));
}
static UniValue signrawtransactionwithkey(const JSONRPCRequest& request)
{
    if (request.fHelp || request.params.size() < 2 || request.params.size() > 4)
        throw std::runtime_error(
            RPCHelpMan{"signrawtransactionwithkey",
                "\nSign inputs for raw transaction (serialized, hex-encoded).\n"
                "The second argument is an array of base58-encoded private\n"
                "keys that will be the only keys used to sign the transaction.\n"
                "The third optional argument (may be null) is an array of previous transaction outputs that\n"
                "this transaction depends on but may not yet be in the block chain.\n",
                {
                    {"hexstring", RPCArg::Type::STR, RPCArg::Optional::NO, "The transaction hex string"},
                    {"privkeys", RPCArg::Type::ARR, RPCArg::Optional::NO, "A json array of base58-encoded private keys for signing",
                        {
                            {"privatekey", RPCArg::Type::STR_HEX, RPCArg::Optional::OMITTED, "private key in base58-encoding"},
                        },
                        },
                    {"prevtxs", RPCArg::Type::ARR, RPCArg::Optional::OMITTED_NAMED_ARG, "A json array of previous dependent transaction outputs",
                        {