`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(!ParseFixedPoint(".1", 8, &amount));
    BOOST_CHECK(!ParseFixedPoint("--0.1", 8, &amount));
    BOOST_CHECK(!ParseFixedPoint("0.000000001", 8, &amount));
    BOOST_CHECK(!ParseFixedPoint("-0.000000001", 8, &amount));
    BOOST_CHECK(!ParseFixedPoint("0.00000001000000001", 8, &amount));
    BOOST_CHECK(!ParseFixedPoint("-10000000000.00000000", 8, &amount));
    BOOST_CHECK(!ParseFixedPoint("10000000000.00000000", 8, &amount));
    BOOST_CHECK(!ParseFixedPoint("-10000000000.00000001", 8, &amount));
    BOOST_CHECK(!ParseFixedPoint("10000000000.00000001", 8, &amount));
    BOOST_CHECK(!ParseFixedPoint("-10000000000.00000009", 8, &amount));
    BOOST_CHECK(!ParseFixedPoint("10000000000.00000009", 8, &amount));
    BOOST_CHECK(!ParseFixedPoint("-99999999999.99999999", 8, &amount));
    BOOST_CHECK(!ParseFixedPoint("99999909999.09999999", 8, &amount));
    BOOST_CHECK(!ParseFixedPoint("92233720368.54775807", 8, &amount));
    BOOST_CHECK(!ParseFixedPoint("92233720368.54775808", 8, &amount));
    BOOST_CHECK(!ParseFixedPoint("-92233720368.54775808", 8, &amount));
    BOOST_CHECK(!ParseFixedPoint("-92233720368.54775809", 8, &amount));
    BOOST_CHECK(!ParseFixedPoint("1.1e", 8, &amount));
    BOOST_CHECK(!ParseFixedPoint("1.1e-", 8, &amount));
    BOOST_CHECK(!ParseFixedPoint("1.", 8, &amount));
}
static void TestOtherThread(fs::path dirname, std::string lockname, bool *result)
{
    *result = LockDirectory(dirname, lockname);
}
#ifndef WIN32 
static constexpr char LockCommand = 'L';
static constexpr char UnlockCommand = 'U';
static constexpr char ExitCommand = 'X';
static void TestOtherProcess(fs::path dirname, std::string lockname, int fd)