向输出文件写入文件头信息。
2.8 av_write_trailer() /** * Write the stream trailer to an output media file and free the * file private data. * * May only be called after a successful call to avformat_write_header. * * @param s media file handle * @return 0 if OK, AVERROR_xxx on error */ int av_write_trailer(AVFormatContext *s);向输出文件文件尾信息。