VideoFileWriter::Close() incorrectly deallocates the data->FormatContext structure.

According to ffmpeg's code documentation the AVFormatContext should be free'd with libffmpeg::avformat_free_context(AVFormatContext *s)

The entire structure end related memory is deallocated by avformat_free_context except for data->FormatContext->pb, which should be free'd by libffmpeg::avio_close(*) as shown in the attached patch

Using avformat_free_context() instead of av_free() to properly free FFmpeg's format context.

Committed in revision 1734.