fix crash when decoder is NULL

src/decoder/vorbis_decoder_plugin.c

@@ -79,7 +79,7 @@ static int ogg_seek_cb(void *data, ogg_int64_t offset, int whence)
 	struct vorbis_input_stream *vis = data;
 
 	return vis->seekable &&
-		decoder_get_command(vis->decoder) != DECODE_COMMAND_STOP &&
+		(!vis->decoder || decoder_get_command(vis->decoder) != DECODE_COMMAND_STOP) &&
 		input_stream_seek(vis->input_stream, offset, whence, NULL)
 		? 0 : -1;
 }