`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 std::set<int64_t> i64s{i64, static_cast<int64_t>(u64)};
        const size_t dynamic_usage = memusage::DynamicUsage(i64s);
        const size_t incremental_dynamic_usage = memusage::IncrementalDynamicUsage(i64s);
        assert(dynamic_usage == incremental_dynamic_usage * i64s.size());
    }
    (void)MillisToTimeval(i64);
    const double d = ser_uint64_to_double(u64);
    assert(ser_double_to_uint64(d) == u64);
    const float f = ser_uint32_to_float(u32);
    assert(ser_float_to_uint32(f) == u32);
    (void)SighashToStr(uch);
    (void)SipHashUint256(u64, u64, u256);
    (void)SipHashUint256Extra(u64, u64, u256, u32);
    (void)ToLower(ch);
    (void)ToUpper(ch);
    
    if (i64 != std::numeric_limits<int64_t>::min()) {
        int64_t parsed_money;
        if (ParseMoney(ValueFromAmount(i64).getValStr(), parsed_money)) {
            assert(parsed_money == i64);
        }
    }
    if (i32 >= 0 && i32 <= 16) {
        assert(i32 == CScript::DecodeOP_N(CScript::EncodeOP_N(i32)));
    }
    const std::chrono::seconds seconds{i64};
    assert(count_seconds(seconds) == i64);
    const CScriptNum script_num{i64};
    (void)script_num.getint();