crypto/cipher: use AEAD.NonceSize to make nonce in the example

The existing example uses hard-coded constant to make nonce buffer.
Using AEAD.NonceSize makes it a more portable and appropriate example.

Fixes: #48372

Change-Id: I7c7a38ed48aff46ca11ef4f5654c778eac13dde6
GitHub-Last-Rev: 03ccbb1
GitHub-Pull-Request: #48373
Reviewed-on: https://go-review.googlesource.com/c/go/+/349603
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Sean Liao <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Auto-Submit: Sean Liao <[email protected]>
Reviewed-by: Roland Shoemaker <[email protected]>

Author: najeira

src/crypto/cipher/example_test.go

@@ -29,14 +29,14 @@ func ExampleNewGCM_encrypt() {
 		panic(err.Error())
 	}
 
-	// Never use more than 2^32 random nonces with a given key because of the risk of a repeat.
-	nonce := make([]byte, 12)
-	if _, err := io.ReadFull(rand.Reader, nonce); err != nil {
+	aesgcm, err := cipher.NewGCM(block)
+	if err != nil {
 		panic(err.Error())
 	}
 
-	aesgcm, err := cipher.NewGCM(block)
-	if err != nil {
+	// Never use more than 2^32 random nonces with a given key because of the risk of a repeat.
+	nonce := make([]byte, aesgcm.NonceSize())
+	if _, err := io.ReadFull(rand.Reader, nonce); err != nil {
 		panic(err.Error())
 	}