avcodec/mpeg4videodec: Check for bitstream end in read_quant_matrix_ext()

Fixes: out of array read
Fixes: asff-crash-0e53d0dc491dfdd507530b66562812fbd4c36678

Found-by: Paul Ch <paulcher@icloud.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
static av_cold int decode_end(AVCodecContext *avctx)
{
    Mpeg4DecContext *ctx = avctx->priv_data;
    int i;

    if (!avctx->internal->is_copy) {
        for (i = 0; i < 12; i++)
            ff_free_vlc(&ctx->studio_intra_tab[i]);

        ff_free_vlc(&ctx->studio_luma_dc);
        ff_free_vlc(&ctx->studio_chroma_dc);
    }

    return ff_h263_decode_end(avctx);
}

static av_cold int decode_end(AVCodecContext *avctx)
{
    Mpeg4DecContext *ctx = avctx->priv_data;
    int i;

    if (!avctx->internal->is_copy) {
        for (i = 0; i < 12; i++)
            ff_free_vlc(&ctx->studio_intra_tab[i]);

        ff_free_vlc(&ctx->studio_luma_dc);
        ff_free_vlc(&ctx->studio_chroma_dc);
    }

    return ff_h263_decode_end(avctx);
}
