`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.
BOOST_CHECK(!ParseMoney(" ", ret));
    BOOST_CHECK(!ParseMoney("  ", ret));
    
    BOOST_CHECK(!ParseMoney("1 2", ret));
    BOOST_CHECK(!ParseMoney(" 1 2 ", ret));
    BOOST_CHECK(!ParseMoney(" 1.2 3 ", ret));
    BOOST_CHECK(!ParseMoney(" 1 2.3 ", ret));
    
    BOOST_CHECK(!ParseMoney("92233720368.54775808", ret));
    
    BOOST_CHECK(!ParseMoney("-1", ret));
    
    BOOST_CHECK(!ParseMoney("\0-1"s, ret));
    BOOST_CHECK(!ParseMoney("\0" "1"s, ret));
    BOOST_CHECK(!ParseMoney("1\0"s, ret));
}
BOOST_AUTO_TEST_CASE(util_IsHex)
{
    BOOST_CHECK(IsHex("00"));
    BOOST_CHECK(IsHex("00112233445566778899aabbccddeeffAABBCCDDEEFF"));
    BOOST_CHECK(IsHex("ff"));
    BOOST_CHECK(IsHex("FF"));
    BOOST_CHECK(!IsHex(""));
    BOOST_CHECK(!IsHex("0"));
    BOOST_CHECK(!IsHex("a"));
    BOOST_CHECK(!IsHex("eleven"));
    BOOST_CHECK(!IsHex("00xx00"));
    BOOST_CHECK(!IsHex("0x0000"));
}
BOOST_AUTO_TEST_CASE(util_IsHexNumber)