DecoderThread: set Decoder::song_tag only for local files

If the song tag comes from a stream, and MPD playback restarts, MPD
would believe the tag should override the newly received tag.  This
makes the previous tag appear stuck.  This change passes the song tag
only if it's authoritative - i.e. if it's a song file.

NEWS

@@ -4,6 +4,7 @@ ver 0.19.10 (not yet released)
   - smbclient: fix DFF playback
 * decoder
   - ffmpeg: improve seeking accuracy
+  - fix stuck stream tags
 * encoder
   - opus: fix bogus granulepos
 * output

src/decoder/DecoderThread.cxx

@@ -380,7 +380,11 @@ decoder_run_song(DecoderControl &dc,
 		 const DetachedSong &song, const char *uri, Path path_fs)
 {
 	Decoder decoder(dc, dc.start_time.IsPositive(),
-			new Tag(song.GetTag()));
+			/* pass the song tag only if it's
+			   authoritative, i.e. if it's a local file -
+			   tags on "stream" songs are just remembered
+			   from the last time we played it*/
+			song.IsFile() ? new Tag(song.GetTag()) : nullptr);
 	int ret;
 
 	dc.state = DecoderState::START;