<!-- 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. -->

{
    if (request.fHelp || request.params.size() != 1)
        throw std::runtime_error(
            RPCHelpMan{"decodepsbt",
                "\nReturn a JSON object representing the serialized, base64-encoded partially signed Bitcoin transaction.\n",
                {
                    {"psbt", RPCArg::Type::STR, RPCArg::Optional::NO, "The PSBT base64 string"},
                },
                RPCResult{
            "{\n"
            "  \"tx\" : {                   (json object) The decoded network-serialized unsigned transaction.\n"
            "    ...                                      The layout is the same as the output of decoderawtransaction.\n"
            "  },\n"
            "  \"unknown\" : {                (json object) The unknown global fields\n"
            "    \"key\" : \"value\"            (key-value pair) An unknown key-value pair\n"
            "     ...\n"
            "  },\n"
            "  \"inputs\" : [                 (array of json objects)\n"
            "    {\n"
            "      \"non_witness_utxo\" : {   (json object, optional) Decoded network transaction for non-witness UTXOs\n"
            "        ...\n"
            "      },\n"
            "      \"witness_utxo\" : {            (json object, optional) Transaction output for witness UTXOs\n"
            "        \"amount\" : x.xxx,           (numeric) The value in " + CURRENCY_UNIT + "\n"
            "        \"scriptPubKey\" : {          (json object)\n"
            "          \"asm\" : \"asm\",            (string) The asm\n"