decoder/Reader: change Decoder pointer to reference

src/decoder/Reader.cxx

@@ -24,5 +24,5 @@
 size_t
 DecoderReader::Read(void *data, size_t size)
 {
-	return decoder_read(decoder, is, data, size);
+	return decoder_read(&decoder, is, data, size);
 }

src/decoder/Reader.hxx

@@ -32,14 +32,11 @@ class InputStream;
  * interface.
  */
 class DecoderReader final : public Reader {
-	Decoder *const decoder;
+	Decoder &decoder;
 	InputStream &is;
 
 public:
 	DecoderReader(Decoder &_decoder, InputStream &_is)
-		:decoder(&_decoder), is(_is) {}
-
-	DecoderReader(Decoder *_decoder, InputStream &_is)
 		:decoder(_decoder), is(_is) {}
 
 	/* virtual methods from class Reader */