`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.
action == NEGATE_TEST  ? "-notestnet=1" :
                                              action == ENABLE_REG   ? "-regtest=1"   :
                                              action == DISABLE_REG  ? "-regtest=0"   :
                                              action == NEGATE_REG   ? "-noregtest=1" : nullptr; };
        std::string desc;
        std::vector<const char*> argv = {"ignored"};
        for (Action action : arg_actions) {
            const char* argstr = arg(action);
            if (!argstr) break;
            argv.push_back(argstr);
            desc += " ";
            desc += argv.back();
        }
        std::string error;
        BOOST_CHECK(parser.ParseParameters(argv.size(), argv.data(), error));
        BOOST_CHECK_EQUAL(error, "");
        std::string conf;
        for (Action action : conf_actions) {
            const char* argstr = arg(action);
            if (!argstr) break;
            desc += " ";
            desc += argstr + 1;
            conf += argstr + 1;
            conf += "\n";
        }
        std::istringstream conf_stream(conf);
        BOOST_CHECK(parser.ReadConfigStream(conf_stream, "filepath", error));
        BOOST_CHECK_EQUAL(error, "");
        desc += " || ";
        try {