CCBC-857: fix possible NULL pointer dereference in mcreq_reserve_key
CCBC-857: fix possible NULL pointer dereference in mcreq_reserve_key

Change-Id: If5f60ea98e95499fb5d8cccbd8a23fb82708e396
Reviewed-on: http://review.couchbase.org/84220
Tested-by: Build Bot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Sergey Avseyev <87f6d5e4fd3644c3c20800cde7fd3ad1569370b3@gmail.com>

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