Recover from afs_GetVolSlot errors
afs_GetVolSlot can panic in a few different ways, such as failing to
read from or write to VolumeInfo. Instead of panic'ing, return an
error to the application. Adjust callers to deal with getting a NULL
volume returned.
Based on a patch by Mike Meffie.
Change-Id: Ibb301fed795ee9ee5906b8e7973945a06218b8f1
Reviewed-on: http://gerrit.openafs.org/1801
Tested-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>