`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.
try {
        int nVersion;
        ds_mtx >> nVersion;
        ds_mtx.SetVersion(nVersion);
        ds_mtx >> mutable_tx;
    } catch (const std::ios_base::failure&) {
        valid_mutable_tx = false;
    }
    assert(valid_tx == valid_mutable_tx);
    if (!valid_tx) {
        return;
    }
    TxValidationState state_with_dupe_check;
    (void)CheckTransaction(tx, state_with_dupe_check);
    const CFeeRate dust_relay_fee{DUST_RELAY_TX_FEE};
    std::string reason;
    const bool is_standard_with_permit_bare_multisig = IsStandardTx(tx,  true, dust_relay_fee, reason);
    const bool is_standard_without_permit_bare_multisig = IsStandardTx(tx,  false, dust_relay_fee, reason);
    if (is_standard_without_permit_bare_multisig) {
        assert(is_standard_with_permit_bare_multisig);
    }
    (void)tx.GetHash();
    (void)tx.GetTotalSize();
    try {
        (void)tx.GetValueOut();
    } catch (const std::runtime_error&) {
    }
    (void)tx.GetWitnessHash();
    (void)tx.HasWitness();
    (void)tx.IsCoinBase();