mpeg12dec: move setting first_field to mpeg_field_start()

For field picture, the first_field is set based on its previous value.
Before this commit, first_field is set when reading the picture
coding extension. However, in corrupted files there may be multiple
picture coding extension headers, so the final value of first_field that
is actually used during decoding can be wrong. That can lead to various
undefined behaviour, like predicting from a non-existing field.

Fix this problem, by setting first_field in mpeg_field_start(), which
should be called exactly once per field.

CC: libav-stable@libav.org
Bug-ID: 999
