static int lowest_referenced_row(MpegEncContext *s, int dir)
{
    int my_max = INT_MIN, my_min = INT_MAX, qpel_shift = !s->quarter_sample;
    int my, off, i, mvs;

    if (s->picture_structure != PICT_FRAME || s->mcsel)
        goto unhandled;

    switch (s->mv_type) {
        case MV_TYPE_16X16:
            mvs = 1;
            break;
        case MV_TYPE_16X8:
            mvs = 2;
            break;
        case MV_TYPE_8X8:
            mvs = 4;
            break;
        default:
            goto unhandled;
    }

    for (i = 0; i < mvs; i++) {
        my = s->mv[dir][i][1];
        my_max = FFMAX(my_max, my);
        my_min = FFMIN(my_min, my);
    }

    off = ((FFMAX(-my_min, my_max)<<qpel_shift) + 63) >> 6;

    return av_clip(s->mb_y + off, 0, s->mb_height - 1);
unhandled:
    return s->mb_height-1;
}

The product dereferences a pointer that it expects to be valid but is NULL.