`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.
(void)tx.IsNull();
    (void)tx.ToString();
    (void)EncodeHexTx(tx);
    (void)GetLegacySigOpCount(tx);
    (void)GetTransactionWeight(tx);
    (void)GetVirtualTransactionSize(tx);
    (void)IsFinalTx(tx,  1024,  1024);
    (void)IsStandardTx(tx, reason);
    (void)RecursiveDynamicUsage(tx);
    (void)SignalsOptInRBF(tx);
    CCoinsView coins_view;
    const CCoinsViewCache coins_view_cache(&coins_view);
    (void)AreInputsStandard(tx, coins_view_cache, false);
    (void)AreInputsStandard(tx, coins_view_cache, true);
    (void)IsWitnessStandard(tx, coins_view_cache);
    UniValue u(UniValue::VOBJ);
    
    bool skip_tx_to_univ = false;
    for (const CTxOut& txout : tx.vout) {
        if (txout.nValue == std::numeric_limits<int64_t>::min()) {
            skip_tx_to_univ = true;
        }
    }
    if (!skip_tx_to_univ) {
        TxToUniv(tx,  {}, u);
        static const uint256 u256_max(uint256S("ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff"));
        TxToUniv(tx, u256_max, u);
    }
}