math/big: call norm when returning success from Rat SetString

After CL 24430, reflect.DeepEqual no longer returns true when comparing
a *Rat built with (*Rat).SetString("0") with one built with
(*Rat).SetInt64(0).
These should be equivalent, but because (*Rat).SetString does not call
norm() when returning the zero value, the result of reflect.DeepEqual
will be false.

One could suggest that developers should use (*Rat).Cmp instead
of relying on reflect.DeepEqual, but if a (*Rat) is part of a
larger struct that is being compared, this can be cumbersome.

This is fixed by calling z.norm() when returning zero in SetString.

Fixes #50944

Change-Id: Ib84ae975bf82fe02d1203aa9668a01960c0fd59d
Reviewed-on: https://go-review.googlesource.com/c/go/+/364434
Reviewed-by: Katie Hockman <katie@golang.org>
Trust: Katie Hockman <katie@golang.org>
Trust: Ian Lance Taylor <iant@golang.org>

src/math/big/ratconv.go

@@ -113,7 +113,7 @@ func (z *Rat) SetString(s string) (*Rat, bool) {
 
 	// special-case 0 (see also issue #16176)
 	if len(z.a.abs) == 0 {
-		return z, true
+		return z.norm(), true
 	}
 	// len(z.a.abs) > 0
 

src/math/big/ratconv_test.go

@@ -9,6 +9,7 @@ import (
 	"fmt"
 	"io"
 	"math"
+	"reflect"
 	"strconv"
 	"strings"
 	"testing"
@@ -205,6 +206,14 @@ func TestRatSetString(t *testing.T) {
 	}
 }
 
+func TestRatSetStringZero(t *testing.T) {
+	got, _ := new(Rat).SetString("0")
+	want := new(Rat).SetInt64(0)
+	if !reflect.DeepEqual(got, want) {
+		t.Errorf("got %#+v, want %#+v", got, want)
+	}
+}
+
 func TestRatScan(t *testing.T) {
 	var buf bytes.Buffer
 	for i, test := range setStringTests {