When running `samtools idxstats` on an indexed BAM file `foo.bam` in order to build a list of chromosomes, I get a result that includes a line I'm not sure how to interpret:

```
$ samtools idxstats foo.bam | cut -f1
chr1
chr10
chr11
...
chrY
*
```

What is the asterisk supposed to refer to, when I iterate through this list and extract data from that chromosome?
in the code of `bam_index.c`

    printf("*\t0\t0\t%llu\n", (long long)idx->n_no_coor);

where `n_no_coor` = "unmapped reads without coordinate"