`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(ValueFromAmount(2099999999999999LL).write() == "20999999.99999999");
    BOOST_CHECK_EQUAL(ValueFromAmount(0).write(), "0.00000000");
    BOOST_CHECK_EQUAL(ValueFromAmount((COIN/10000)*123456789).write(), "12345.67890000");
    BOOST_CHECK_EQUAL(ValueFromAmount(-COIN).write(), "-1.00000000");
    BOOST_CHECK_EQUAL(ValueFromAmount(-COIN/10).write(), "-0.10000000");
    BOOST_CHECK_EQUAL(ValueFromAmount(COIN*100000000).write(), "100000000.00000000");
    BOOST_CHECK_EQUAL(ValueFromAmount(COIN*10000000).write(), "10000000.00000000");
    BOOST_CHECK_EQUAL(ValueFromAmount(COIN*1000000).write(), "1000000.00000000");
    BOOST_CHECK_EQUAL(ValueFromAmount(COIN*100000).write(), "100000.00000000");
    BOOST_CHECK_EQUAL(ValueFromAmount(COIN*10000).write(), "10000.00000000");
    BOOST_CHECK_EQUAL(ValueFromAmount(COIN*1000).write(), "1000.00000000");
    BOOST_CHECK_EQUAL(ValueFromAmount(COIN*100).write(), "100.00000000");
    BOOST_CHECK_EQUAL(ValueFromAmount(COIN*10).write(), "10.00000000");
    BOOST_CHECK_EQUAL(ValueFromAmount(COIN).write(), "1.00000000");
    BOOST_CHECK_EQUAL(ValueFromAmount(COIN/10).write(), "0.10000000");
    BOOST_CHECK_EQUAL(ValueFromAmount(COIN/100).write(), "0.01000000");
    BOOST_CHECK_EQUAL(ValueFromAmount(COIN/1000).write(), "0.00100000");
    BOOST_CHECK_EQUAL(ValueFromAmount(COIN/10000).write(), "0.00010000");
    BOOST_CHECK_EQUAL(ValueFromAmount(COIN/100000).write(), "0.00001000");
    BOOST_CHECK_EQUAL(ValueFromAmount(COIN/1000000).write(), "0.00000100");
    BOOST_CHECK_EQUAL(ValueFromAmount(COIN/10000000).write(), "0.00000010");
    BOOST_CHECK_EQUAL(ValueFromAmount(COIN/100000000).write(), "0.00000001");
}
static UniValue ValueFromString(const std::string &str)
{
    UniValue value;
    BOOST_CHECK(value.setNumStr(str));
    return value;
}
BOOST_AUTO_TEST_CASE(rpc_parse_monetary_values)