DecoderAPI: discard unused song tag early

If there's a stream tag, don't let the song tag override it in the
next update_stream_tag() call.

src/decoder/DecoderAPI.cxx

@@ -433,8 +433,11 @@ update_stream_tag(Decoder &decoder, InputStream *is)
 
 		/* no stream tag present - submit the song tag
 		   instead */
-		decoder.song_tag = nullptr;
-	}
+	} else
+		/* discard the song tag; we don't need it */
+		delete decoder.song_tag;
+
+	decoder.song_tag = nullptr;
 
 	delete decoder.stream_tag;
 	decoder.stream_tag = tag;