int ff_mpv_common_frame_size_change(MpegEncContext *s)
{
    int i, err = 0;

    if (!s->context_initialized)
        return AVERROR(EINVAL);

    if (s->slice_context_count > 1) {
        for (i = 0; i < s->slice_context_count; i++) {
            free_duplicate_context(s->thread_context[i]);
        }
        for (i = 1; i < s->slice_context_count; i++) {
            av_freep(&s->thread_context[i]);
        }
    } else
        free_duplicate_context(s);

    free_context_frame(s);

    if (s->picture)
        for (i = 0; i < MAX_PICTURE_COUNT; i++) {
                s->picture[i].needs_realloc = 1;
        }

    s->last_picture_ptr         =
    s->next_picture_ptr         =
    s->current_picture_ptr      = NULL;

    if (s->codec_id == AV_CODEC_ID_MPEG2VIDEO && !s->progressive_sequence)
        s->mb_height = (s->height + 31) / 32 * 2;
    else
        s->mb_height = (s->height + 15) / 16;

    if ((s->width || s->height) &&
        (err = av_image_check_size(s->width, s->height, 0, s->avctx)) < 0)
        goto fail;

    if ((err = init_context_frame(s)))
        goto fail;

    memset(s->thread_context, 0, sizeof(s->thread_context));
    s->thread_context[0]   = s;

    if (s->width && s->height) {
        int nb_slices = s->slice_context_count;
        if (nb_slices > 1) {
            for (i = 0; i < nb_slices; i++) {
                if (i) {
                    s->thread_context[i] = av_memdup(s, sizeof(MpegEncContext));
                    if (!s->thread_context[i]) {
                        err = AVERROR(ENOMEM);
                        goto fail;
                    }
                }
                if ((err = init_duplicate_context(s->thread_context[i])) < 0)
                    goto fail;
                s->thread_context[i]->start_mb_y =
                    (s->mb_height * (i) + nb_slices / 2) / nb_slices;
                s->thread_context[i]->end_mb_y   =
                    (s->mb_height * (i + 1) + nb_slices / 2) / nb_slices;
            }
        } else {
            err = init_duplicate_context(s);
            if (err < 0)
                goto fail;
            s->start_mb_y = 0;
            s->end_mb_y   = s->mb_height;
        }
        s->slice_context_count = nb_slices;
    }

    return 0;
 fail:
    ff_mpv_common_end(s);
    return err;
}

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