Despite the doxy stating that it's called when the muxer is destroyed,
this was not true in practice. It's only called by av_write_trailer()
and on init() failure.

An AVFormatContext may be closed without writing the trailer if errors
ocurred while muxing packets, so in order to prevent memory leaks, it
should effectively be called when freeing the muxer.
Signed-off-by: James Almer <jamrial@gmail.com>