sgidec: use unchecked bytestream2 functions where it makes sense

Overreads can not happen as header size of 512 bytes is required.

Signed-off-by: Paul B Mahol <onemda@gmail.com>
Handle an invalid extra mpeg2 picture header following a frame-encoded picture.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>