`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.
const uint32_t deserialized_u32 = ser_readdata32(stream);
        assert(u32 == deserialized_u32 && stream.empty());
        ser_writedata32be(stream, u32);
        const uint32_t deserialized_u32be = ser_readdata32be(stream);
        assert(u32 == deserialized_u32be && stream.empty());
        ser_writedata16(stream, u16);
        const uint16_t deserialized_u16 = ser_readdata16(stream);
        assert(u16 == deserialized_u16 && stream.empty());
        ser_writedata16be(stream, u16);
        const uint16_t deserialized_u16be = ser_readdata16be(stream);
        assert(u16 == deserialized_u16be && stream.empty());
        ser_writedata8(stream, u8);
        const uint8_t deserialized_u8 = ser_readdata8(stream);
        assert(u8 == deserialized_u8 && stream.empty());
    }
    {
        CDataStream stream(SER_NETWORK, INIT_PROTO_VERSION);
        WriteCompactSize(stream, u64);
        try {
            const uint64_t deserialized_u64 = ReadCompactSize(stream);
            assert(u64 == deserialized_u64 && stream.empty());
        } catch (const std::ios_base::failure&) {
        }
    }
    try {
        CHECK_NONFATAL(b);
    } catch (const NonFatalCheckError&) {
    }
}