media: fix memory leaks

avcodec_close leaks memory, while avcodec_free_context doesn't.

FFmpeg documentation specifies not to use avcodec_close, as opening and
closing a context multiple times is no longer supported.

Change-Id: I230891f2510c355b9d44d918dcf387399d79a8aa
Reviewed-by: Sebastien Blin <sebastien.blin@savoirfairelinux.com>