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
},
            }.ToString());
    
    
    pwallet->BlockUntilSyncedToCurrentChain();
    auto locked_chain = pwallet->chain().lock();
    LOCK(pwallet->cs_wallet);
    RPCTypeCheckArgument(request.params[0], UniValue::VBOOL);
    bool fUnlock = request.params[0].get_bool();
    if (request.params[1].isNull()) {
        if (fUnlock)
            pwallet->UnlockAllCoins();
        return true;
    }
    RPCTypeCheckArgument(request.params[1], UniValue::VARR);
    const UniValue& output_params = request.params[1];
    
    std::vector<COutPoint> outputs;
    outputs.reserve(output_params.size());
    for (unsigned int idx = 0; idx < output_params.size(); idx++) {
        const UniValue& o = output_params[idx].get_obj();
        RPCTypeCheckObj(o,
            {
                {"txid", UniValueType(UniValue::VSTR)},
                {"vout", UniValueType(UniValue::VNUM)},
            });
        const uint256 txid(ParseHashO(o, "txid"));
        const int nOutput = find_value(o, "vout").get_int();
        if (nOutput < 0) {
            throw JSONRPCError(RPC_INVALID_PARAMETER, "Invalid parameter, vout must be positive");