`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.
void ReadConfigString(const std::string str_config)
    {
        std::istringstream streamConfig(str_config);
        {
            LOCK(cs_args);
            m_settings.ro_config.clear();
            m_config_sections.clear();
        }
        std::string error;
        BOOST_REQUIRE(ReadConfigStream(streamConfig, "", error));
    }
    void SetNetworkOnlyArg(const std::string arg)
    {
        LOCK(cs_args);
        m_network_only_args.insert(arg);
    }
    void SetupArgs(const std::vector<std::pair<std::string, unsigned int>>& args)
    {
        for (const auto& arg : args) {
            AddArg(arg.first, "", arg.second, OptionsCategory::OPTIONS);
        }
    }
    using ArgsManager::GetSetting;
    using ArgsManager::GetSettingsList;
    using ArgsManager::ReadConfigStream;
    using ArgsManager::cs_args;
    using ArgsManager::m_network;
    using ArgsManager::m_settings;
};
class CheckValueTest : public TestChain100Setup