encoding/json: make BenchmarkSkipValue more consistent

Move scanner allocation out of loop.
It's the only allocation in the test so it dominates
when it triggers a garbage collection.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/5369117

src/pkg/encoding/json/scanner_test.go

@@ -186,11 +186,12 @@ func TestNextValueBig(t *testing.T) {
 	}
 }
 
+var benchScan scanner
+
 func BenchmarkSkipValue(b *testing.B) {
 	initBig()
-	var scan scanner
 	for i := 0; i < b.N; i++ {
-		nextValue(jsonBig, &scan)
+		nextValue(jsonBig, &benchScan)
 	}
 	b.SetBytes(int64(len(jsonBig)))
 }