libroot/posix: fchdir() should not accept AT_FDCWD.

It doesn't make sense to change directories to the current one.

Should fix #19059.

src/system/libroot/posix/unistd/directory.c

@@ -23,6 +23,8 @@ chdir(const char *path)
 int
 fchdir(int fd)
 {
+	if (fd < 0)
+		RETURN_AND_SET_ERRNO(EBADF);
 	RETURN_AND_SET_ERRNO(_kern_setcwd(fd, NULL));
 }