MB-11305 Use correct type for sizeof() when allocating hashtable.

Change-Id: I3cf0a46699bd30241bd9708b1e9bc82084e6a7cb
Reviewed-on: http://review.couchbase.org/37836
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>