mpegvideo_enc: Fix encoding videos with less frames than the delay of the encoder

mpegvideo_enc: Fix encoding videos with less frames than the delay of the encoder

When the encoder is fed with less frames than its delay, the picture list
looks like { NULL, NULL, ..., frame, frame, frame }. When flushing the
encoder (input frame == NULL), we need to ensure the picture list is
shifted enough so that we do not return an empty packet, which would
mean the encoder has finished, while it has not encoded any frame.

Signed-off-by: Vittorio Giovara <vittorio.giovara@gmail.com>