`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.
int64_t parsed_money;
        if (ParseMoney(FormatMoney(i64), parsed_money)) {
            assert(parsed_money == i64);
        }
    }
    (void)GetSizeOfCompactSize(u64);
    (void)GetSpecialScriptSize(u32);
    if (!MultiplicationOverflow(i64, static_cast<int64_t>(::nBytesPerSigOp)) && !AdditionOverflow(i64 * ::nBytesPerSigOp, static_cast<int64_t>(4))) {
        (void)GetVirtualTransactionSize(i64, i64);
    }
    if (!MultiplicationOverflow(i64, static_cast<int64_t>(u32)) && !AdditionOverflow(i64, static_cast<int64_t>(4)) && !AdditionOverflow(i64 * u32, static_cast<int64_t>(4))) {
        (void)GetVirtualTransactionSize(i64, i64, u32);
    }
    (void)HexDigit(ch);
    (void)MoneyRange(i64);
    (void)ToString(i64);
    (void)IsDigit(ch);
    (void)IsSpace(ch);
    (void)IsSwitchChar(ch);
    (void)memusage::DynamicUsage(ch);
    (void)memusage::DynamicUsage(i16);
    (void)memusage::DynamicUsage(i32);
    (void)memusage::DynamicUsage(i64);
    (void)memusage::DynamicUsage(i8);
    (void)memusage::DynamicUsage(u16);
    (void)memusage::DynamicUsage(u32);
    (void)memusage::DynamicUsage(u64);
    (void)memusage::DynamicUsage(u8);
    const unsigned char uch = static_cast<unsigned char>(u8);
    (void)memusage::DynamicUsage(uch);