Merge commit '54dd9b1c'

* commit '54dd9b1c':
  h264: set mb_aff_frame in frame_start()
  h264: move the block starting a new field out of slice_header_parse()

Both commits are merged at the same time in order to prevent a
regression with Ticket #4440 (see 38660128).
Merged-by: Clément Bœsch <u@pkh.me>