`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.
int processstatus;
    BOOST_CHECK_EQUAL(write(fd[1], &LockCommand, 1), 1);
    BOOST_CHECK_EQUAL(write(fd[1], &ExitCommand, 1), 1);
    BOOST_CHECK_EQUAL(waitpid(pid, &processstatus, 0), pid);
    BOOST_CHECK_EQUAL(processstatus, 0);
    BOOST_CHECK_EQUAL(LockDirectory(dirname, lockname, true), true);
    
    signal(SIGCHLD, old_handler);
    BOOST_CHECK_EQUAL(close(fd[1]), 0); 
#endif
    
    ReleaseDirectoryLocks();
    fs::remove_all(dirname);
}
BOOST_AUTO_TEST_CASE(test_DirIsWritable)
{
    
    fs::path tmpdirname = GetDataDir();
    BOOST_CHECK_EQUAL(DirIsWritable(tmpdirname), true);
    
    tmpdirname = GetUniquePath(tmpdirname);
    BOOST_CHECK_EQUAL(DirIsWritable(tmpdirname), false);
    fs::create_directory(tmpdirname);
    
    BOOST_CHECK_EQUAL(DirIsWritable(tmpdirname), true);
    fs::remove(tmpdirname);
}
BOOST_AUTO_TEST_CASE(test_ToLower)