avformat/mxfdec: Fix av_log context

Fixes: out of array access
Fixes: mxf-crash-1c2e59bf07a34675bfb3ada5e1ec22fa9f38f923

Found-by: Paul Ch <paulcher@icloud.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
static MXFIndexTable *mxf_find_index_table(MXFContext *mxf, int index_sid)
{
    int i;
    for (i = 0; i < mxf->nb_index_tables; i++)
        if (mxf->index_tables[i].index_sid == index_sid)
            return &mxf->index_tables[i];
    return NULL;
}

static MXFIndexTable *mxf_find_index_table(MXFContext *mxf, int index_sid)
{
    int i;
    for (i = 0; i < mxf->nb_index_tables; i++)
        if (mxf->index_tables[i].index_sid == index_sid)
            return &mxf->index_tables[i];
    return NULL;
}
