Update some nits

Change-Id: I5164549afe6997f4512c9dc1954b97ca7a2c6c4d

google/internal/externalaccount/filecredsource.go

@@ -9,6 +9,7 @@ import (
 	"encoding/json"
 	"errors"
 	"fmt"
+	"io"
 	"io/ioutil"
 	"os"
 )
@@ -24,7 +25,7 @@ func (cs fileCredentialSource) subjectToken() (string, error) {
 		return "", fmt.Errorf("oauth2/google: failed to open credential file %q", cs.File)
 	}
 	defer tokenFile.Close()
-	tokenBytes, err := ioutil.ReadAll(tokenFile)
+	tokenBytes, err := ioutil.ReadAll(io.LimitReader(tokenFile, 1<<20))
 	if err != nil {
 		return "", fmt.Errorf("oauth2/google: failed to read credential file: %v", err)
 	}