json: fix test when rand returns 0.

Fixes using gccgo when optimizing, which changes the order of
calls to rand.  Same patch proposed upstream.

From-SVN: r175927

libgo/go/json/scanner_test.go

@@ -252,6 +252,9 @@ func genArray(n int) []interface{} {
 	if f > n {
 		f = n
 	}
+	if n > 0 && f == 0 {
+		f = 1
+	}
 	x := make([]interface{}, int(f))
 	for i := range x {
 		x[i] = genValue(((i+1)*n)/f - (i*n)/f)