media: ffmpeg: Remove AsMut from AvFrame.
media: ffmpeg: Remove AsMut from AvFrame.

This is actually highly unsound since it allows you to write arbitrary
garbage to the buffer pointer fields.

BUG=None
TEST=cargo test -p ffmpeg &&
     cargo test --features "video-decoder,ffmpeg" -p devices video
     + v4l2r test from crosvm book

Change-Id: I77bd32b856afef8538791a99829f7248cd77787e
Reviewed-on: https://chromium-review.googlesource.com/c/crosvm/crosvm/+/3822433
Tested-by: Tatsuyuki Ishi <9fb0affb047566c1ca29d70049b0f579b84f88dc@google.com>
Reviewed-by: Alexandre Courbot <301cd6a3ea50a894b4b51e385c5eb12b9a4ffc0c@chromium.org>
Commit-Queue: Tatsuyuki Ishi <9fb0affb047566c1ca29d70049b0f579b84f88dc@google.com>

google/crosvm,google/crosvm,google/crosvm,google/crosvm