Mpq::ratio is unsafe

[Rufflewind]

Mpq::ratio is unsafe:

extern crate gmp;
use gmp::mpq::Mpq;
use gmp::mpz::Mpz;
fn main() {
    println!("{:?}", Mpq::ratio(&Mpz::one(), &-Mpz::one()));
    //                                        ^--- (!)
}

results in:

zsh: segmentation fault (core dumped)

Tested this using gmp-6.1.2 on Arch Linux.

To quote the manual:

Note that if an assignment to the numerator and/or denominator could take an mpq_t out of the canonical form described at the start of this chapter (see Rational Number Functions) then mpq_canonicalize must be called before any other mpq functions are applied to that mpq_t.

There are probably other rational functions that need to be scrutinized as well.

[vks]

Mpq::from_str_radix is also affected. #22 should fix this.