MacOS: panic decoder should check for unloaded kexts

MacOS: panic decoder should check for unloaded kexts

if the openafs kext was unloaded from under us, try to handle that.

Reviewed-on: http://gerrit.openafs.org/3683
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit 9743a644adb37230eae3b11ad3e4a1e8e7a1ebce)

Change-Id: Iaa0ac75de42424411ea66f04afdf0e8303aa4adc
Reviewed-on: http://gerrit.openafs.org/3839
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>