decoder/flac: moved decoder initialization to _flac_common.c

Invoke decoder_initialized() in the libFLAC metadata callback.  This
merges code from the FLAC and the OggFLAC decoder plugin into the
common library.

src/decoder/oggflac_decoder_plugin.c

@@ -297,7 +297,7 @@ oggflac_decode(struct decoder * mpd_decoder, struct input_stream *input_stream)
 		goto fail;
 	}
 
-	if (!flac_data_get_audio_format(&data, &audio_format))
+	if (!data.initialized)
 		goto fail;
 
 	decoder_initialized(mpd_decoder, &audio_format,