`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.
{
public:
    UniValue CallRPC(std::string args);
};
UniValue RPCTestingSetup::CallRPC(std::string args)
{
    std::vector<std::string> vArgs;
    boost::split(vArgs, args, boost::is_any_of(" \t"));
    std::string strMethod = vArgs[0];
    vArgs.erase(vArgs.begin());
    util::Ref context{m_node};
    JSONRPCRequest request(context);
    request.strMethod = strMethod;
    request.params = RPCConvertValues(strMethod, vArgs);
    request.fHelp = false;
    if (RPCIsInWarmup(nullptr)) SetRPCWarmupFinished();
    try {
        UniValue result = tableRPC.execute(request);
        return result;
    }
    catch (const UniValue& objError) {
        throw std::runtime_error(find_value(objError, "message").get_str());
    }
}
BOOST_FIXTURE_TEST_SUITE(rpc_tests, RPCTestingSetup)
BOOST_AUTO_TEST_CASE(rpc_rawparams)