internal/stream: reject trailing data (no EOF) after end of stream

tests/stream_trailing_garbage_long.go

@@ -0,0 +1,22 @@
+// Copyright 2022 The age Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build ignore
+
+package main
+
+import "filippo.io/age/internal/testkit"
+
+func main() {
+	f := testkit.NewTestFile()
+	f.FileKey(testkit.LargeTestFileKey)
+	f.VersionLine("v1")
+	f.X25519(testkit.TestX25519Identity)
+	f.HMAC()
+	f.Nonce(testkit.LargeTestNonce)
+	f.PayloadChunkFinal(testkit.LargeTestFirstChunk)
+	f.Buf.Write(f.Rand(20))
+	f.ExpectPartialPayload(64 * 1024)
+	f.Generate()
+}