`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.
stream >> deserialized_i16;
        assert(i16 == deserialized_i16 && stream.empty());
        uint8_t deserialized_u8;
        stream << u8;
        stream >> deserialized_u8;
        assert(u8 == deserialized_u8 && stream.empty());
        int8_t deserialized_i8;
        stream << i8;
        stream >> deserialized_i8;
        assert(i8 == deserialized_i8 && stream.empty());
        char deserialized_ch;
        stream << ch;
        stream >> deserialized_ch;
        assert(ch == deserialized_ch && stream.empty());
        bool deserialized_b;
        stream << b;
        stream >> deserialized_b;
        assert(b == deserialized_b && stream.empty());
    }
    {
        const ServiceFlags service_flags = (ServiceFlags)u64;
        (void)HasAllDesirableServiceFlags(service_flags);
        (void)MayHaveUsefulAddressDB(service_flags);
    }
    {
        CDataStream stream(SER_NETWORK, INIT_PROTO_VERSION);
        ser_writedata64(stream, u64);
        const uint64_t deserialized_u64 = ser_readdata64(stream);
        assert(u64 == deserialized_u64 && stream.empty());
        ser_writedata32(stream, u32);