check for cwd() failures

The web server produced 9 GB of this before filling up the disk:

```
Use of uninitialized value $path in stat at /gsc/scripts/opt/genome/snapshots/genome-3545/lib/perl/above.pm line 35.
Use of uninitialized value $dev in numeric eq (==) at /gsc/scripts/opt/genome/snapshots/genome-3545/lib/perl/above.pm line 59.
```