`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_EQUAL(FormatMoney(0), "0.00");
    BOOST_CHECK_EQUAL(FormatMoney((COIN/10000)*123456789), "12345.6789");
    BOOST_CHECK_EQUAL(FormatMoney(-COIN), "-1.00");
    BOOST_CHECK_EQUAL(FormatMoney(COIN*100000000), "100000000.00");
    BOOST_CHECK_EQUAL(FormatMoney(COIN*10000000), "10000000.00");
    BOOST_CHECK_EQUAL(FormatMoney(COIN*1000000), "1000000.00");
    BOOST_CHECK_EQUAL(FormatMoney(COIN*100000), "100000.00");
    BOOST_CHECK_EQUAL(FormatMoney(COIN*10000), "10000.00");
    BOOST_CHECK_EQUAL(FormatMoney(COIN*1000), "1000.00");
    BOOST_CHECK_EQUAL(FormatMoney(COIN*100), "100.00");
    BOOST_CHECK_EQUAL(FormatMoney(COIN*10), "10.00");
    BOOST_CHECK_EQUAL(FormatMoney(COIN), "1.00");
    BOOST_CHECK_EQUAL(FormatMoney(COIN/10), "0.10");
    BOOST_CHECK_EQUAL(FormatMoney(COIN/100), "0.01");
    BOOST_CHECK_EQUAL(FormatMoney(COIN/1000), "0.001");
    BOOST_CHECK_EQUAL(FormatMoney(COIN/10000), "0.0001");
    BOOST_CHECK_EQUAL(FormatMoney(COIN/100000), "0.00001");
    BOOST_CHECK_EQUAL(FormatMoney(COIN/1000000), "0.000001");
    BOOST_CHECK_EQUAL(FormatMoney(COIN/10000000), "0.0000001");
    BOOST_CHECK_EQUAL(FormatMoney(COIN/100000000), "0.00000001");
}
BOOST_AUTO_TEST_CASE(util_ParseMoney)