Fix builds with NDEBUG

Now that cb_assert won't optimize away the expression we
need the inFreeList method

Change-Id: Idcaf18b83b25091bd14cd558f3a7a98055ed3b02
Reviewed-on: http://review.couchbase.org/34777
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>