`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.
BOOST_CHECK(test_args.IsArgSet("-ccc"));
    BOOST_CHECK(test_args.IsArgSet("-d"));
    BOOST_CHECK(test_args.IsArgSet("-fff"));
    BOOST_CHECK(test_args.IsArgSet("-ggg"));
    BOOST_CHECK(test_args.IsArgSet("-h"));
    BOOST_CHECK(test_args.IsArgSet("-i"));
    BOOST_CHECK(!test_args.IsArgSet("-zzz"));
    BOOST_CHECK(!test_args.IsArgSet("-iii"));
    BOOST_CHECK_EQUAL(test_args.GetArg("-a", "xxx"), "");
    BOOST_CHECK_EQUAL(test_args.GetArg("-b", "xxx"), "1");
    BOOST_CHECK_EQUAL(test_args.GetArg("-ccc", "xxx"), "argument");
    BOOST_CHECK_EQUAL(test_args.GetArg("-d", "xxx"), "e");
    BOOST_CHECK_EQUAL(test_args.GetArg("-fff", "xxx"), "0");
    BOOST_CHECK_EQUAL(test_args.GetArg("-ggg", "xxx"), "1");
    BOOST_CHECK_EQUAL(test_args.GetArg("-h", "xxx"), "0");
    BOOST_CHECK_EQUAL(test_args.GetArg("-i", "xxx"), "1");
    BOOST_CHECK_EQUAL(test_args.GetArg("-zzz", "xxx"), "xxx");
    BOOST_CHECK_EQUAL(test_args.GetArg("-iii", "xxx"), "xxx");
    for (const bool def : {false, true}) {
        BOOST_CHECK(test_args.GetBoolArg("-a", def));
        BOOST_CHECK(test_args.GetBoolArg("-b", def));
        BOOST_CHECK(!test_args.GetBoolArg("-ccc", def));
        BOOST_CHECK(!test_args.GetBoolArg("-d", def));
        BOOST_CHECK(!test_args.GetBoolArg("-fff", def));
        BOOST_CHECK(test_args.GetBoolArg("-ggg", def));
        BOOST_CHECK(!test_args.GetBoolArg("-h", def));
        BOOST_CHECK(test_args.GetBoolArg("-i", def));
        BOOST_CHECK(test_args.GetBoolArg("-zzz", def) == def);
        BOOST_CHECK(test_args.GetBoolArg("-iii", def) == def);
    }