If the demuxer doesn't set extradata in the stream's codecpar, a
need_context_update request will delete the previously extracted extradata in
the stream's internal AVCodecContext.
As we can't ensure the old extradata is valid for the stream in its post
context update request state, try to get extradata from the new packet instead
of attempting to preserve the old in some form.
Signed-off-by: James Almer <jamrial@gmail.com>