relnote: check error from io.ReadAll

Change-Id: Ibf075a5f643eab6ba29ff8798f6e2bd8eb30ec4d
Reviewed-on: https://go-review.googlesource.com/c/build/+/644155
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Auto-Submit: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>

relnote/relnote.go

@@ -462,5 +462,8 @@ func checkFragmentFile(fsys fs.FS, filename string) error {
 	}
 	defer f.Close()
 	data, err := io.ReadAll(f)
+	if err != nil {
+		return err
+	}
 	return CheckFragment(string(data))
 }