avformat/asfdec_o: Check size_bmp more fully

Fixes: integer overflow and out of array access
Fixes: asfo-crash-46080c4341572a7137a162331af77f6ded45cbd7

Found-by: Paul Ch <paulcher@icloud.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
static AVStream *find_stream(AVFormatContext *s, uint16_t st_num)
{
    AVStream *st = NULL;
    ASFContext *asf = s->priv_data;
    int i;

    for (i = 0; i < asf->nb_streams; i++) {
        if (asf->asf_st[i]->stream_index == st_num) {
            st = s->streams[asf->asf_st[i]->index];
            break;
        }
    }

    return st;
}

static AVStream *find_stream(AVFormatContext *s, uint16_t st_num)
{
    AVStream *st = NULL;
    ASFContext *asf = s->priv_data;
    int i;

    for (i = 0; i < asf->nb_streams; i++) {
        if (asf->asf_st[i]->stream_index == st_num) {
            st = s->streams[asf->asf_st[i]->index];
            break;
        }
    }

    return st;
}
