libafs: don't call afs_PutDCache(NULL) in afs_GetDownD()

libafs: don't call afs_PutDCache(NULL) in afs_GetDownD()

It's possible for an eviction candidate to be omitted by the small
for loop (around line 670), leaving its reference in victimDCs
set to NULL.  In the big for loop that follows, don't call
afs_PutDCache() when we hit one.

Found-by: clang static analyzer with help from AFS_NONNULL
Reviewed-on: http://gerrit.openafs.org/5260
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit 10d27341808be41b29fbcc09b8bd7523c3b7a541)

Change-Id: I4ff3ab9a57c1c48be0f7533ab3adb1e4b6ababf9
Reviewed-on: http://gerrit.openafs.org/5263
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>