<!-- Describe the issue -->
`getrawtransaction` fails
<!--- What behavior did you expect? -->
Return decoded raw tx
<!--- What was the actual behavior (provide screenshots if the issue is GUI-related)? -->
* `bitcoin-qt -prune=8192  -server -rpcuser=rpc -rpcpassword=rpc -onlynet=ipv4 -rpcbind=127.0.0.1:18332 -testnet`
  * BTC 0.18.0
  * `getrawtransaction b423b2b22d6d74376de2887c07f7c84ae890a93d541fcc6dbf3686f2923b2753 1`
![image](https://user-images.githubusercontent.com/3670331/58841021-658d7180-8668-11e9-9f9f-ed1f794070ea.png)
  * BTC 0.17.1
  * `getrawtransaction b423b2b22d6d74376de2887c07f7c84ae890a93d541fcc6dbf3686f2923b2753 1`
![image](https://user-images.githubusercontent.com/3670331/58841089-9ff70e80-8668-11e9-8a68-5022631b43d7.png)

<!--- How reliably can you reproduce the issue, what are the steps to do so? -->
Request `getrawtransaction`.
<!-- What version of Bitcoin Core are you using, where did you get it (website, self-compiled, etc)? -->
Official from site.
<!-- What type of machine are you observing the error on (OS/CPU and disk type)? -->
`Linux pentagon 4.9.0-9-amd64 #1 SMP Debian 4.9.168-1+deb9u2 (2019-05-13) x86_64 GNU/Linux`, u5200, 2x8Gb, SSD
<!-- For the GUI-related issue on Linux provide names and versions of a distro, a desktop environment and a graphical shell (if relevant). -->
QT
<!-- Any extra information that might be useful in the debugging process. -->
<!--- This is normally the contents of a `debug.log` or `config.log` file. Raw text or a link to a pastebin type site are preferred. -->

"      \"unknown\" : {                (json object) The unknown global fields\n"
            "        \"key\" : \"value\"            (key-value pair) An unknown key-value pair\n"
            "         ...\n"
            "      },\n"
            "    }\n"
            "    ,...\n"
            "  ]\n"
            "  \"fee\" : fee                      (numeric, optional) The transaction fee paid if all UTXOs slots in the PSBT have been filled.\n"
            "}\n"
                },
                RPCExamples{
                    HelpExampleCli("decodepsbt", "\"psbt\"")
                },
            }.ToString());
    RPCTypeCheck(request.params, {UniValue::VSTR});
    
    PartiallySignedTransaction psbtx;
    std::string error;
    if (!DecodeBase64PSBT(psbtx, request.params[0].get_str(), error)) {
        throw JSONRPCError(RPC_DESERIALIZATION_ERROR, strprintf("TX decode failed %s", error));
    }
    UniValue result(UniValue::VOBJ);
    
    UniValue tx_univ(UniValue::VOBJ);
    TxToUniv(CTransaction(*psbtx.tx), uint256(), tx_univ, false);
    result.pushKV("tx", tx_univ);
    
    UniValue unknowns(UniValue::VOBJ);
    for (auto entry : psbtx.unknown) {
        unknowns.pushKV(HexStr(entry.first), HexStr(entry.second));