static void dct_unquantize_mpeg2_intra_c(MpegEncContext *s,
                                   int16_t *block, int n, int qscale)
{
    int i, level, nCoeffs;
    const uint16_t *quant_matrix;

    if (s->q_scale_type) qscale = ff_mpeg2_non_linear_qscale[qscale];
    else                 qscale <<= 1;

    if(s->alternate_scan) nCoeffs= 63;
    else nCoeffs= s->block_last_index[n];

    block[0] *= n < 4 ? s->y_dc_scale : s->c_dc_scale;
    quant_matrix = s->intra_matrix;
    for(i=1;i<=nCoeffs;i++) {
        int j= s->intra_scantable.permutated[i];
        level = block[j];
        if (level) {
            if (level < 0) {
                level = -level;
                level = (int)(level * qscale * quant_matrix[j]) >> 4;
                level = -level;
            } else {
                level = (int)(level * qscale * quant_matrix[j]) >> 4;
            }
            block[j] = level;
        }
    }
}

The product dereferences a pointer that it expects to be valid but is NULL.