Cast mc_ssize_t to ssize_t

Change-Id: Ie8fa31ef74eebe558aaa9f5c6488ef5aa7af552e
Reviewed-on: http://review.couchbase.org/29351
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>