strings,bytes: use result of copy in subsequent slicing

This can get rid of a bounds check.
Followup to CL 622240.

Change-Id: I9d0a2c0408b8d274c46136d32d7a5fb09b4aad1c
Reviewed-on: https://go-review.googlesource.com/c/go/+/622955
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

src/bytes/buffer.go

@@ -247,8 +247,8 @@ func growSlice(b []byte, n int) []byte {
 		c = 2 * cap(b)
 	}
 	b2 := append([]byte(nil), make([]byte, c)...)
-	copy(b2, b)
-	return b2[:len(b)]
+	i := copy(b2, b)
+	return b2[:i]
 }
 
 // WriteTo writes data to w until the buffer is drained or an error occurs.