custr_remove
return (custr_remove(cus, idx, len));
int custr_remove(custr_t *, size_t, size_t);
FAIL(custr_remove(cu, 6, 2), EINVAL);
FAIL(custr_remove(cu, 2, 10), EINVAL);
VERIFY0(custr_remove(cu, 0, 1));
VERIFY0(custr_remove(cu, 1, 2));