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 int mxf_metadataset_init(MXFMetadataSet *ctx, enum MXFMetadataSetType type)
{
    switch (type){
    case MultipleDescriptor:
    case Descriptor:
        ((MXFDescriptor*)ctx)->pix_fmt = AV_PIX_FMT_NONE;
        ((MXFDescriptor*)ctx)->duration = AV_NOPTS_VALUE;
        break;
    default:
        break;
    }
    return 0;
}

static int mxf_metadataset_init(MXFMetadataSet *ctx, enum MXFMetadataSetType type)
{
    switch (type){
    case MultipleDescriptor:
    case Descriptor:
        ((MXFDescriptor*)ctx)->pix_fmt = AV_PIX_FMT_NONE;
        ((MXFDescriptor*)ctx)->duration = AV_NOPTS_VALUE;
        break;
    default:
        break;
    }
    return 0;
}
