audio/vorbis: test Length with a non-seekable source

Updates #2252

audio/vorbis/vorbis.go

@@ -44,6 +44,8 @@ func (s *Stream) Seek(offset int64, whence int) (int64, error) {
 }
 
 // Length returns the size of decoded stream in bytes.
+//
+// If the source is not io.Seeker, Length returns 0.
 func (s *Stream) Length() int64 {
 	return s.size
 }

audio/vorbis/vorbis_test.go

@@ -83,5 +83,10 @@ func TestNonSeeker(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
-	_ = s
+
+	got := s.Length()
+	want := int64(0)
+	if got != want {
+		t.Errorf("s.Length(): got: %d, want: %d", got, want)
+	}
 }