`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.
{
    TestArgsManager test;
    test.SetupArgs({{"-registered", ArgsManager::ALLOW_ANY}});
    const char* argv[] = {"ignored", "-registered"};
    std::string error;
    BOOST_CHECK(test.ParseParameters(2, (char**)argv, error));
    BOOST_CHECK_EQUAL(error, "");
    argv[1] = "-unregistered";
    BOOST_CHECK(!test.ParseParameters(2, (char**)argv, error));
    BOOST_CHECK_EQUAL(error, "Invalid parameter -unregistered");
    
    
    argv[1] = "-test.registered";
    BOOST_CHECK(!test.ParseParameters(2, (char**)argv, error));
    BOOST_CHECK_EQUAL(error, "Invalid parameter -test.registered");
}
static void TestParse(const std::string& str, bool expected_bool, int64_t expected_int)
{
    TestArgsManager test;
    test.SetupArgs({{"-value", ArgsManager::ALLOW_ANY}});
    std::string arg = "-value=" + str;
    const char* argv[] = {"ignored", arg.c_str()};
    std::string error;
    BOOST_CHECK(test.ParseParameters(2, (char**)argv, error));
    BOOST_CHECK_EQUAL(test.GetBoolArg("-value", false), expected_bool);
    BOOST_CHECK_EQUAL(test.GetBoolArg("-value", true), expected_bool);
    BOOST_CHECK_EQUAL(test.GetArg("-value", 99998), expected_int);
    BOOST_CHECK_EQUAL(test.GetArg("-value", 99999), expected_int);
}
BOOST_AUTO_TEST_CASE(util_ArgParsing)