`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.
CAmount result[NUM_GETBLOCKSTATS_PERCENTILES] = { 0 };
    for (int64_t i = 0; i < 100; i++) {
        feerates.emplace_back(std::make_pair(1 ,1));
    }
    for (int64_t i = 0; i < 100; i++) {
        feerates.emplace_back(std::make_pair(2 ,1));
    }
    CalculatePercentilesByWeight(result, feerates, total_weight);
    BOOST_CHECK_EQUAL(result[0], 1);
    BOOST_CHECK_EQUAL(result[1], 1);
    BOOST_CHECK_EQUAL(result[2], 1);
    BOOST_CHECK_EQUAL(result[3], 2);
    BOOST_CHECK_EQUAL(result[4], 2);
    
    total_weight = 100;
    CAmount result2[NUM_GETBLOCKSTATS_PERCENTILES] = { 0 };
    feerates.clear();
    feerates.emplace_back(std::make_pair(1, 9));
    feerates.emplace_back(std::make_pair(2 , 16)); 
    feerates.emplace_back(std::make_pair(4 ,50)); 
    feerates.emplace_back(std::make_pair(5 ,10));
    feerates.emplace_back(std::make_pair(9 ,15));  
    CalculatePercentilesByWeight(result2, feerates, total_weight);
    BOOST_CHECK_EQUAL(result2[0], 2);
    BOOST_CHECK_EQUAL(result2[1], 2);
    BOOST_CHECK_EQUAL(result2[2], 4);
    BOOST_CHECK_EQUAL(result2[3], 4);
    BOOST_CHECK_EQUAL(result2[4], 9);
    
    total_weight = 100;