`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(IsHexNumber("0x0"));
    BOOST_CHECK(IsHexNumber("0"));
    BOOST_CHECK(IsHexNumber("0x10"));
    BOOST_CHECK(IsHexNumber("10"));
    BOOST_CHECK(IsHexNumber("0xff"));
    BOOST_CHECK(IsHexNumber("ff"));
    BOOST_CHECK(IsHexNumber("0xFfa"));
    BOOST_CHECK(IsHexNumber("Ffa"));
    BOOST_CHECK(IsHexNumber("0x00112233445566778899aabbccddeeffAABBCCDDEEFF"));
    BOOST_CHECK(IsHexNumber("00112233445566778899aabbccddeeffAABBCCDDEEFF"));
    BOOST_CHECK(!IsHexNumber(""));   
    BOOST_CHECK(!IsHexNumber("0x")); 
    BOOST_CHECK(!IsHexNumber("0x0 ")); 
    BOOST_CHECK(!IsHexNumber(" 0x0")); 
    BOOST_CHECK(!IsHexNumber("0x 0")); 
    BOOST_CHECK(!IsHexNumber(" "));    
    BOOST_CHECK(!IsHexNumber("0x0ga")); 
    BOOST_CHECK(!IsHexNumber("x0"));    
    BOOST_CHECK(!IsHexNumber("0x0x00")); 
}
BOOST_AUTO_TEST_CASE(util_seed_insecure_rand)