static void gmc1_motion(MpegEncContext *s,
                        uint8_t *dest_y, uint8_t *dest_cb, uint8_t *dest_cr,
                        uint8_t **ref_picture)
{
    uint8_t *ptr;
    int src_x, src_y, motion_x, motion_y;
    ptrdiff_t offset, linesize, uvlinesize;
    int emu = 0;

    motion_x   = s->sprite_offset[0][0];
    motion_y   = s->sprite_offset[0][1];
    src_x      = s->mb_x * 16 + (motion_x >> (s->sprite_warping_accuracy + 1));
    src_y      = s->mb_y * 16 + (motion_y >> (s->sprite_warping_accuracy + 1));
    motion_x <<= (3 - s->sprite_warping_accuracy);
    motion_y <<= (3 - s->sprite_warping_accuracy);
    src_x      = av_clip(src_x, -16, s->width);
    if (src_x == s->width)
        motion_x = 0;
    src_y = av_clip(src_y, -16, s->height);
    if (src_y == s->height)
        motion_y = 0;

    linesize   = s->linesize;
    uvlinesize = s->uvlinesize;

    ptr = ref_picture[0] + src_y * linesize + src_x;

    if ((unsigned)src_x >= FFMAX(s->h_edge_pos - 17, 0) ||
        (unsigned)src_y >= FFMAX(s->v_edge_pos - 17, 0)) {
        s->vdsp.emulated_edge_mc(s->sc.edge_emu_buffer, ptr,
                                 linesize, linesize,
                                 17, 17,
                                 src_x, src_y,
                                 s->h_edge_pos, s->v_edge_pos);
        ptr = s->sc.edge_emu_buffer;
    }

    if ((motion_x | motion_y) & 7) {
        s->mdsp.gmc1(dest_y, ptr, linesize, 16,
                     motion_x & 15, motion_y & 15, 128 - s->no_rounding);
        s->mdsp.gmc1(dest_y + 8, ptr + 8, linesize, 16,
                     motion_x & 15, motion_y & 15, 128 - s->no_rounding);
    } else {
        int dxy;

        dxy = ((motion_x >> 3) & 1) | ((motion_y >> 2) & 2);
        if (s->no_rounding) {
            s->hdsp.put_no_rnd_pixels_tab[0][dxy](dest_y, ptr, linesize, 16);
        } else {
            s->hdsp.put_pixels_tab[0][dxy](dest_y, ptr, linesize, 16);
        }
    }

    if (CONFIG_GRAY && s->avctx->flags & AV_CODEC_FLAG_GRAY)
        return;

    motion_x   = s->sprite_offset[1][0];
    motion_y   = s->sprite_offset[1][1];
    src_x      = s->mb_x * 8 + (motion_x >> (s->sprite_warping_accuracy + 1));
    src_y      = s->mb_y * 8 + (motion_y >> (s->sprite_warping_accuracy + 1));
    motion_x <<= (3 - s->sprite_warping_accuracy);
    motion_y <<= (3 - s->sprite_warping_accuracy);
    src_x      = av_clip(src_x, -8, s->width >> 1);
    if (src_x == s->width >> 1)
        motion_x = 0;
    src_y = av_clip(src_y, -8, s->height >> 1);
    if (src_y == s->height >> 1)
        motion_y = 0;

    offset = (src_y * uvlinesize) + src_x;
    ptr    = ref_picture[1] + offset;
    if ((unsigned)src_x >= FFMAX((s->h_edge_pos >> 1) - 9, 0) ||
        (unsigned)src_y >= FFMAX((s->v_edge_pos >> 1) - 9, 0)) {
        s->vdsp.emulated_edge_mc(s->sc.edge_emu_buffer, ptr,
                                 uvlinesize, uvlinesize,
                                 9, 9,
                                 src_x, src_y,
                                 s->h_edge_pos >> 1, s->v_edge_pos >> 1);
        ptr = s->sc.edge_emu_buffer;
        emu = 1;
    }
    s->mdsp.gmc1(dest_cb, ptr, uvlinesize, 8,
                 motion_x & 15, motion_y & 15, 128 - s->no_rounding);

    ptr = ref_picture[2] + offset;
    if (emu) {
        s->vdsp.emulated_edge_mc(s->sc.edge_emu_buffer, ptr,
                                 uvlinesize, uvlinesize,
                                 9, 9,
                                 src_x, src_y,
                                 s->h_edge_pos >> 1, s->v_edge_pos >> 1);
        ptr = s->sc.edge_emu_buffer;
    }
    s->mdsp.gmc1(dest_cr, ptr, uvlinesize, 8,
                 motion_x & 15, motion_y & 15, 128 - s->no_rounding);
}

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