big: fix (*Rat) SetFrac64(a, b) when b < 0.

R=gri
CC=golang-dev
https://golang.org/cl/3352041

src/pkg/big/rat.go

@@ -35,9 +35,8 @@ func (z *Rat) SetFrac(a, b *Int) *Rat {
 func (z *Rat) SetFrac64(a, b int64) *Rat {
 	z.a.SetInt64(a)
 	if b < 0 {
-		z.b.setUint64(uint64(-b))
+		b = -b
 		z.a.neg = !z.a.neg
-		return z.norm()
 	}
 	z.b = z.b.setUint64(uint64(b))
 	return z.norm()

src/pkg/big/rat_test.go

@@ -257,3 +257,26 @@ func TestIssue820(t *testing.T) {
 		t.Errorf("got %s want %s", z, q)
 	}
 }
+
+
+var setFrac64Tests = []struct {
+	a, b int64
+	out  string
+}{
+	{0, 1, "0"},
+	{0, -1, "0"},
+	{1, 1, "1"},
+	{-1, 1, "-1"},
+	{1, -1, "-1"},
+	{-1, -1, "1"},
+	{-9223372036854775808, -9223372036854775808, "1"},
+}
+
+func TestRatSetFrac64Rat(t *testing.T) {
+	for i, test := range setFrac64Tests {
+		x := new(Rat).SetFrac64(test.a, test.b)
+		if x.RatString() != test.out {
+			t.Errorf("#%d got %s want %s", i, x.RatString(), test.out)
+		}
+	}
+}