unhandled exception: pair_depth == 0
Brent;
Thanks again for helping get mosdepth packaged and ready to go. I've done some initial integration work in bcbio and started testing on larger inputs and ran into an assertion error rated to pair_depth:
```
mosdepth -t 2 -F 1804 -b /mnt/work/NA12878-hg38-val/work/bedprep/cov-cov-cov-Exome-AZ.bed -d /mnt/work/NA12878-hg38-val/work/bcbiotx/tmpxFqJGS/cov-cov-cov-Exome-AZ-distribution.txt /mnt/work/NA12878-hg38-val/work/align/NA12878-1/NA12878-1-sort.bam > /mnt/work/NA12878-hg38-val/work/bcbiotx/tmpRCOY_q/cov-cov-cov-Exome-AZ-coverage.bed
Traceback (most recent call last)
mosdepth.nim(435)        mosdepth
mosdepth.nim(240)        window_main
system.nim(3554)         failedAssertImpl
system.nim(3546)         raiseAssert
system.nim(2656)         sysFatal
Error: unhandled exception: pair_depth == 0  [AssertionError]
```
This is publicly available data so I can try to isolate to a test case but it's a big whole genome run so am not sure about the best way to narrow it down on a first glance. If just the traceback is not useful I'll look into slicing and dicing the BED to try and provide a reproducible case. Thanks again.