[coverity] vbucket.c: fix NULL checks for capabilities
[coverity] vbucket.c: fix NULL checks for capabilities

Change-Id: I8a614a196b515cb4ffc238e9ed76e79b3fc11ab4
Reviewed-on: http://review.couchbase.org/122365
Tested-by: Build Bot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Sergey Avseyev <87f6d5e4fd3644c3c20800cde7fd3ad1569370b3@gmail.com>

couchbase/libcouchbase,kojiromike/libcouchbase,couchbase/libcouchbase,couchbase/libcouchbase,couchbase/libcouchbase,kojiromike/libcouchbase,couchbase/libcouchbase,couchbase/libcouchbase,couchbase/libcouchbase,kojiromike/libcouchbase,kojiromike/libcouchbase