`FormatMoney(CAmount n)` is not being properly defined for `n = std::numeric_limits<CAmount>::min()`. This is due to an invalid integer negation: the negation of `-9223372036854775808` cannot be represented in type `CAmount` (which has a max value of `9223372036854775807`):

https://github.com/bitcoin/bitcoin/blob/54f812d9d29893c690ae06b84aaeab128186aa36/src/util/moneystr.cpp#L12-L31

FWIW `FormatMoney(-9223372036854775808)` returns `--92233720368.-54775808` when compiled with UBSan (LLVM/Clang, `-fsanitize=undefined`).

Context: https://github.com/bitcoin/bitcoin/pull/20383#discussion_r523250890

It would be nice if `FormatMoney(CAmount n)` was properly defined for all `CAmount n` :)

These are the "expectations of least surprise":

```
BOOST_CHECK_EQUAL(FormatMoney(std::numeric_limits<CAmount>::max()), "92233720368.54775807");       //  9223372036854775807
BOOST_CHECK_EQUAL(FormatMoney(std::numeric_limits<CAmount>::max() - 1), "92233720368.54775806");   //  9223372036854775806
BOOST_CHECK_EQUAL(FormatMoney(std::numeric_limits<CAmount>::max() - 2), "92233720368.54775805");   //  9223372036854775805
BOOST_CHECK_EQUAL(FormatMoney(std::numeric_limits<CAmount>::max() - 3), "92233720368.54775804");   //  9223372036854775804
BOOST_CHECK_EQUAL(FormatMoney(std::numeric_limits<CAmount>::min() + 3), "-92233720368.54775805");  // -9223372036854775805
BOOST_CHECK_EQUAL(FormatMoney(std::numeric_limits<CAmount>::min() + 2), "-92233720368.54775806");  // -9223372036854775806
BOOST_CHECK_EQUAL(FormatMoney(std::numeric_limits<CAmount>::min() + 1), "-92233720368.54775807");  // -9223372036854775807
BOOST_CHECK_EQUAL(FormatMoney(std::numeric_limits<CAmount>::min()), "-92233720368.54775808");      // -9223372036854775808
```

These assertions are _guaranteed_ to hold for all cases above _except_ for the last one.
desc += network;
        parser.m_network = network;
        const std::string& name = net_specific ? "wallet" : "server";
        const std::string key = "-" + name;
        parser.AddArg(key, name, ArgsManager::ALLOW_ANY, OptionsCategory::OPTIONS);
        if (net_specific) parser.SetNetworkOnlyArg(key);
        auto args = GetValues(arg_actions, section, name, "a");
        std::vector<const char*> argv = {"ignored"};
        for (auto& arg : args) {
            arg.insert(0, "-");
            desc += " ";
            desc += arg;
            argv.push_back(arg.c_str());
        }
        std::string error;
        BOOST_CHECK(parser.ParseParameters(argv.size(), argv.data(), error));
        BOOST_CHECK_EQUAL(error, "");
        std::string conf;
        for (auto& conf_val : GetValues(conf_actions, section, name, "c")) {
            desc += " ";
            desc += conf_val;
            conf += conf_val;
            conf += "\n";
        }
        std::istringstream conf_stream(conf);
        BOOST_CHECK(parser.ReadConfigStream(conf_stream, "filepath", error));
        BOOST_CHECK_EQUAL(error, "");
        if (soft_set) {
            desc += " soft";
            parser.SoftSetArg(key, "soft1");