rat_mul_rat: fix squaring path writing the denominator into the numerator

[louisnovy]

No description provided.

[Kelimion]

Good find. Thanks for the fix and test.

Apropos of the latter, because we know the numerator and denominator are small, we don't need the string conversion:

for r in ([]^big.Rat{&square, &x}) {
	num, _ := big.int_get_i64(&r.a)
	testing.expect_value(t, num, 4)

	den, _ := big.int_get_i64(&r.b)
	testing.expect_value(t, den, 9)
}

Just for future reference. It works fine as-is, so I'll merge it.