mpeg4videodec: use ROUNDED_DIV for pts calculation

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

libavcodec/mpeg4videodec.c

@@ -2007,7 +2007,7 @@ static int decode_vop_header(MpegEncContext *s, GetBitContext *gb){
     }
 
     if(s->avctx->time_base.num)
-        s->current_picture_ptr->f.pts = (s->time + s->avctx->time_base.num / 2) / s->avctx->time_base.num;
+        s->current_picture_ptr->f.pts = ROUNDED_DIV(s->time, s->avctx->time_base.num);
     else
         s->current_picture_ptr->f.pts = AV_NOPTS_VALUE;
     if(s->avctx->debug&FF_DEBUG_PTS)