dafs: avoid null deref getting volume header

dafs: avoid null deref getting volume header

we don't assign hd before dereferencing. stop dereferencing
and just use the queue cursor.

Reviewed-on: http://gerrit.openafs.org/5814
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit 00beae8c4e3b390637f20584e676cf86c817dadd)

Change-Id: Iec2dbbf84e4f08f02998c9e998d2ea899ce48a5d
Reviewed-on: http://gerrit.openafs.org/6122
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>