decoder/opus: fix replay gain when there are no other tags

The `tag_builder.empty()` check was wrong for the SubmitReplayGain()
call.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/497

NEWS

@@ -5,6 +5,8 @@ ver 0.21.6 (not yet released)
   - fix "list" with filter expression
 * input
   - cdio_paranoia: fix build failure due to missing #include
+* decoder
+  - opus: fix replay gain when there are no other tags
 * playlist
   - flac: fix use-after-free bug
 * support abstract sockets on Linux

src/decoder/plugins/OpusDecoderPlugin.cxx

@@ -208,10 +208,12 @@ MPDOpusDecoder::HandleTags(const ogg_packet &packet)
 	TagBuilder tag_builder;
 	AddTagHandler h(tag_builder);
 
-	if (ScanOpusTags(packet.packet, packet.bytes, &rgi, h) &&
-	    !tag_builder.empty()) {
+	if (!ScanOpusTags(packet.packet, packet.bytes, &rgi, h))
+		return;
+
 	client.SubmitReplayGain(&rgi);
 
+	if (!tag_builder.empty()) {
 		Tag tag = tag_builder.Commit();
 		auto cmd = client.SubmitTag(input_stream, std::move(tag));
 		if (cmd != DecoderCommand::NONE)