decoder/opus: allocate buffer only in the first chained song

Fixes memory leak.  That's what we get for

NEWS

@@ -6,6 +6,7 @@ ver 0.21.25 (not yet released)
   - smbclient: don't send credentials to MPD clients
 * decoder
   - opus: apply pre-skip
+  - opus: fix memory leak
 * output
   - osx: improve sample rate selection
 * Windows/Android:

src/decoder/plugins/OpusDecoderPlugin.cxx

@@ -192,6 +192,10 @@ MPDOpusDecoder::OnOggBeginning(const ogg_packet &packet)
 	client.Ready(audio_format, eos_granulepos > 0, duration);
 	frame_size = audio_format.GetFrameSize();
 
+	if (output_buffer == nullptr)
+		/* note: if we ever support changing the channel count
+		   in chained streams, we need to reallocate this
+		   buffer instead of keeping it */
 		output_buffer = new opus_int16[opus_output_buffer_frames
 					       * audio_format.channels];