libafs: Fix directory verification

libafs: Fix directory verification

With the earlier directory verification changes, every directory
was seen as corrupt, because the nde pointer was never being initialised

Rework the way that we check for the failure of afs_dir_GetVerifiedBlob
so that we can more robustly detect problems, whilst still allowing
normal directories to be browsed as before

Change-Id: I3d3f428025296956b5feff6ba290aaef79817dcd
Reviewed-on: http://gerrit.openafs.org/5318
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>