`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.
int64_t now = GetTime();
    BOOST_CHECK(banned_until.get_int64() > now);
    BOOST_CHECK(banned_until.get_int64()-now <= 200);
    
    BOOST_CHECK_THROW(r = CallRPC(std::string("setban 127.0.0.1 add")), std::runtime_error);
    BOOST_CHECK_NO_THROW(CallRPC(std::string("setban 127.0.0.0/24 remove")));
    BOOST_CHECK_NO_THROW(r = CallRPC(std::string("listbanned")));
    ar = r.get_array();
    BOOST_CHECK_EQUAL(ar.size(), 0U);
    BOOST_CHECK_NO_THROW(r = CallRPC(std::string("setban 127.0.0.0/255.255.0.0 add")));
    BOOST_CHECK_THROW(r = CallRPC(std::string("setban 127.0.1.1 add")), std::runtime_error);
    BOOST_CHECK_NO_THROW(CallRPC(std::string("clearbanned")));
    BOOST_CHECK_NO_THROW(r = CallRPC(std::string("listbanned")));
    ar = r.get_array();
    BOOST_CHECK_EQUAL(ar.size(), 0U);
    BOOST_CHECK_THROW(r = CallRPC(std::string("setban test add")), std::runtime_error); 
    
    BOOST_CHECK_NO_THROW(r = CallRPC(std::string("setban FE80:0000:0000:0000:0202:B3FF:FE1E:8329 add")));
    BOOST_CHECK_NO_THROW(r = CallRPC(std::string("listbanned")));
    ar = r.get_array();
    o1 = ar[0].get_obj();
    adr = find_value(o1, "address");
    BOOST_CHECK_EQUAL(adr.get_str(), "fe80::202:b3ff:fe1e:8329/128");
    BOOST_CHECK_NO_THROW(CallRPC(std::string("clearbanned")));
    BOOST_CHECK_NO_THROW(r = CallRPC(std::string("setban 2001:db8::/ffff:fffc:0:0:0:0:0:0 add")));
    BOOST_CHECK_NO_THROW(r = CallRPC(std::string("listbanned")));
    ar = r.get_array();
    o1 = ar[0].get_obj();
    adr = find_value(o1, "address");
    BOOST_CHECK_EQUAL(adr.get_str(), "2001:db8::/30");