bad sin/cos for large angles.

fixes #326.

R=rsc
https://golang.org/cl/160059

src/pkg/math/sin.go

@@ -28,7 +28,7 @@ func sinus(x float64, quad int) float64 {
 		var e float64;
 		e, y = Modf(x);
 		e = e + float64(quad);
-		_, f := Modf(0.25 * e);
+		f, _ := Modf(0.25 * e);
 		quad = int(e - 4*f);
 	} else {
 		k := int32(x);