Use cb_assert instad of assert

Change-Id: Ib175595ac5c59c649a74bb3dc52cc8d62ffc09f5
Reviewed-on: http://review.couchbase.org/43021
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>