encoding/json: fix test failure

$ go test -cpu=1,1,1,1,1,1,1,1,1 encoding/json
--- FAIL: TestIndentBig (0.00 seconds)
        scanner_test.go:131: Indent(jsonBig) did not get bigger
On 4-th run initBig generates an empty array.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/49930051

src/pkg/encoding/json/scanner_test.go

@@ -239,23 +239,16 @@ func trim(b []byte) []byte {
 
 var jsonBig []byte
 
-const (
-	big   = 10000
-	small = 100
-)
-
 func initBig() {
-	n := big
+	n := 10000
 	if testing.Short() {
-		n = small
+		n = 100
 	}
-	if len(jsonBig) != n {
-		b, err := Marshal(genValue(n))
-		if err != nil {
-			panic(err)
-		}
-		jsonBig = b
+	b, err := Marshal(genValue(n))
+	if err != nil {
+		panic(err)
 	}
+	jsonBig = b
 }
 
 func genValue(n int) interface{} {
@@ -296,6 +289,9 @@ func genArray(n int) []interface{} {
 	if f > n {
 		f = n
 	}
+	if f < 1 {
+		f = 1
+	}
 	x := make([]interface{}, f)
 	for i := range x {
 		x[i] = genValue(((i+1)*n)/f - (i*n)/f)