avcodec/mpeg4videodec: Clear bits_per_raw_sample if it has originated from a previous instance

Fixes: assertion failure
Fixes: ffmpeg_crash_5.avi

Found-by: Thuan Pham <thuanpv@comp.nus.edu.sg>, Marcel Böhme, Andrew Santosa and Alexandru Razvan Caciulescu with AFLSmart
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
static int mpeg4_decode_profile_level(MpegEncContext *s, GetBitContext *gb)
{

    s->avctx->profile = get_bits(gb, 4);
    s->avctx->level   = get_bits(gb, 4);

    if (s->avctx->profile == 0 && s->avctx->level == 8) {
        s->avctx->level = 0;
    }

    return 0;
}

static int mpeg4_decode_profile_level(MpegEncContext *s, GetBitContext *gb)
{

    s->avctx->profile = get_bits(gb, 4);
    s->avctx->level   = get_bits(gb, 4);

    if (s->avctx->profile == 0 && s->avctx->level == 8) {
        s->avctx->level = 0;
    }

    return 0;
}
