avcodec/videotoolbox: fix opaque_ref handling

avcodec/videotoolbox: fix opaque_ref handling

This is a partial fix - to fix videotoolbox fully (with multithreaded
hwaccel decoding), much more work would be required.

Before this change, an assertion failure would trigger:

  $ ffmpeg -hwaccel videotoolbox -hwaccel_output_format videotoolbox_vld -i h264.ts -f null -y /dev/null
  Assertion (frame->private_ref && frame->private_ref->size == sizeof(FrameDecodeData)) || !(avctx->codec->capabilities & (1 << 1)) failed at libavcodec/decode.c:620

Signed-off-by: Aman Gupta <aman@tmm1.net>