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 void decode_smpte_tc(Mpeg4DecContext *ctx, GetBitContext *gb)
{
    MpegEncContext *s = &ctx->m;

    skip_bits(gb, 16); /* Time_code[63..48] */
    check_marker(s->avctx, gb, "after Time_code[63..48]");
    skip_bits(gb, 16); /* Time_code[47..32] */
    check_marker(s->avctx, gb, "after Time_code[47..32]");
    skip_bits(gb, 16); /* Time_code[31..16] */
    check_marker(s->avctx, gb, "after Time_code[31..16]");
    skip_bits(gb, 16); /* Time_code[15..0] */
    check_marker(s->avctx, gb, "after Time_code[15..0]");
    skip_bits(gb, 4); /* reserved_bits */
}

static void decode_smpte_tc(Mpeg4DecContext *ctx, GetBitContext *gb)
{
    MpegEncContext *s = &ctx->m;

    skip_bits(gb, 16); /* Time_code[63..48] */
    check_marker(s->avctx, gb, "after Time_code[63..48]");
    skip_bits(gb, 16); /* Time_code[47..32] */
    check_marker(s->avctx, gb, "after Time_code[47..32]");
    skip_bits(gb, 16); /* Time_code[31..16] */
    check_marker(s->avctx, gb, "after Time_code[31..16]");
    skip_bits(gb, 16); /* Time_code[15..0] */
    check_marker(s->avctx, gb, "after Time_code[15..0]");
    skip_bits(gb, 4); /* reserved_bits */
}
