The Matroska muxer up until now leaked memory in two scenarios:

1. If an error happened during writing the trailer, as
mkv_write_trailer() returned early without cleaning up.
2. If mkv_write_header() indicated success despite an error in the
underlying AVIOContext. In this case avformat_write_header() returned
the IO error and according to the API the caller is not allowed to call
av_write_trailer(), so that no cleanup happened for the allocations made
in mkv_write_header().

This has been fixed by using a dedicated deinit function.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
Signed-off-by: James Almer <jamrial@gmail.com>