afs_open: Deal with afs_GetDCache returning NULL

afs_open was assuming afs_GetDCache always returned non-NULL. Make it
deal with a NULL return.

Reviewed-on: http://gerrit.openafs.org/8408
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
(cherry picked from commit 595a614d76d84be602c07d519738556b7f9ebf2a)

Change-Id: Iba54fd03a63be485490db33ff48e5642f55df52d
Reviewed-on: http://gerrit.openafs.org/9500
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>