static inline int hpel_motion_lowres(MpegEncContext *s,
                                     uint8_t *dest, uint8_t *src,
                                     int field_based, int field_select,
                                     int src_x, int src_y,
                                     int width, int height, ptrdiff_t stride,
                                     int h_edge_pos, int v_edge_pos,
                                     int w, int h, h264_chroma_mc_func *pix_op,
                                     int motion_x, int motion_y)
{
    const int lowres   = s->avctx->lowres;
    const int op_index = FFMIN(lowres, 3);
    const int s_mask   = (2 << lowres) - 1;
    int emu = 0;
    int sx, sy;

    if (s->quarter_sample) {
        motion_x /= 2;
        motion_y /= 2;
    }

    sx = motion_x & s_mask;
    sy = motion_y & s_mask;
    src_x += motion_x >> lowres + 1;
    src_y += motion_y >> lowres + 1;

    src   += src_y * stride + src_x;

    if ((unsigned)src_x > FFMAX( h_edge_pos - (!!sx) - w,                 0) ||
        (unsigned)src_y > FFMAX((v_edge_pos >> field_based) - (!!sy) - h, 0)) {
        s->vdsp.emulated_edge_mc(s->sc.edge_emu_buffer, src,
                                 s->linesize, s->linesize,
                                 w + 1, (h + 1) << field_based,
                                 src_x, src_y   << field_based,
                                 h_edge_pos, v_edge_pos);
        src = s->sc.edge_emu_buffer;
        emu = 1;
    }

    sx = (sx << 2) >> lowres;
    sy = (sy << 2) >> lowres;
    if (field_select)
        src += s->linesize;
    pix_op[op_index](dest, src, stride, h, sx, sy);
    return emu;
}

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