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 int decode_new_pred(Mpeg4DecContext *ctx, GetBitContext *gb) {
    MpegEncContext *s = &ctx->m;
    int len = FFMIN(ctx->time_increment_bits + 3, 15);

    get_bits(gb, len);
    if (get_bits1(gb))
        get_bits(gb, len);
    check_marker(s->avctx, gb, "after new_pred");

    return 0;
}

static int decode_new_pred(Mpeg4DecContext *ctx, GetBitContext *gb) {
    MpegEncContext *s = &ctx->m;
    int len = FFMIN(ctx->time_increment_bits + 3, 15);

    get_bits(gb, len);
    if (get_bits1(gb))
        get_bits(gb, len);
    check_marker(s->avctx, gb, "after new_pred");

    return 0;
}
