avformat/mxfdec: Fix DoS issues in mxf_read_index_entry_array()

Fixes: 20170829A.mxf

Co-Author: 张洪亮(望初)" <wangchu.zhl@alibaba-inc.com>
Found-by: Xiaohei and Wangchu from Alibaba Security Team
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
static const MXFCodecUL *mxf_get_codec_ul(const MXFCodecUL *uls, UID *uid)
{
    while (uls->uid[0]) {
        if(mxf_match_uid(uls->uid, *uid, uls->matching_len))
            break;
        uls++;
    }
    return uls;
}

static const MXFCodecUL *mxf_get_codec_ul(const MXFCodecUL *uls, UID *uid)
{
    while (uls->uid[0]) {
        if(mxf_match_uid(uls->uid, *uid, uls->matching_len))
            break;
        uls++;
    }
    return uls;
}
