msmpeg4: fix asm code in ff_msmpeg4_pred_dc()

Fixes first half of Ticket2414

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
msmpeg4: ignore negative DC overflow

Fixes second half of Ticket2414

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
lavc/ass: use bprint API in ff_ass_add_rect().