encoding/json: improve Decode example

Decoding a JSON message does not touch unspecified or null fields;
always use a new underlying struct to prevent old field values from
sticking around.

Fixes: #14640

Change-Id: Ica78c208ce104e2cdee1d4e92bf58596ea5587c8
Reviewed-on: https://go-review.googlesource.com/23483Reviewed-by: Andrew Gerrand <adg@golang.org>

src/encoding/json/example_test.go

@@ -143,10 +143,9 @@ func ExampleDecoder_Decode_stream() {
 	}
 	fmt.Printf("%T: %v\n", t, t)
 
-	var m Message
 	// while the array contains values
 	for dec.More() {
-
+		var m Message
 		// decode an array value (Message)
 		err := dec.Decode(&m)
 		if err != nil {