Fix for #224: Fractional radical simplification

Fix for issue #224.
Can `r` ever be negative? According to the official docs: https://gmplib.org/manual/Integer-Division.html, `r will satisfy 0<=abs(r)<abs(d)`. So I assume that in reality, `-abs(d) < r < abs(d)`, i.e. that `r` could be positive and negative? Is `r` negative for negative rational somehow? Just curious.