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.

Change-Id: Ib0891636a3479bf97cdeab823189e659cb261aa6
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>