Some commands of `bitcoin-tx` accept an index of an existing input/output, e.g. `delin` or `delout`.  And while the index is checked for being out-of-bounds, garbage (non-numeral characters) are happily accepted.  All of the following are fine:

    $ bitcoin-tx -create outaddr=1:1M72Sfpbz1BPpXFHz9m3CdqATR44Jvaydd delout=
    $ bitcoin-tx -create outaddr=1:1M72Sfpbz1BPpXFHz9m3CdqATR44Jvaydd delout=foobar
    $ bitcoin-tx -create outaddr=1:1M72Sfpbz1BPpXFHz9m3CdqATR44Jvaydd delout=0bla

I think all of them should throw an error instead of silently treating the index as zero.

The reason for this is that internally, `atoi` is called - and that by design discards anything after the first non-numeric character (and may just return zero if there are no numerals at all).

A possible fix would be to use `strtol` instead and to verify that the returned `endptr` is actually the end of the input string.  Alternatively, one could convert the integer back to a string and require that it matches the input string exactly.
CMutableTransaction mergedTx{tx};
    const CMutableTransaction txv{tx};
    CCoinsView viewDummy;
    CCoinsViewCache view(&viewDummy);
    if (!registers.count("privatekeys"))
        throw std::runtime_error("privatekeys register variable must be set.");
    CBasicKeyStore tempKeystore;
    UniValue keysObj = registers["privatekeys"];
    for (unsigned int kidx = 0; kidx < keysObj.size(); kidx++) {
        if (!keysObj[kidx].isStr())
            throw std::runtime_error("privatekey not a std::string");
        CKey key = DecodeSecret(keysObj[kidx].getValStr());
        if (!key.IsValid()) {
            throw std::runtime_error("privatekey not valid");
        }
        tempKeystore.AddKey(key);
    }
    
    if (!registers.count("prevtxs"))
        throw std::runtime_error("prevtxs register variable must be set.");
    UniValue prevtxsObj = registers["prevtxs"];
    {
        for (unsigned int previdx = 0; previdx < prevtxsObj.size(); previdx++) {
            UniValue prevOut = prevtxsObj[previdx];
            if (!prevOut.isObject())
                throw std::runtime_error("expected prevtxs internal object");
            std::map<std::string, UniValue::VType> types = {
                {"txid", UniValue::VSTR},