IOCP: Fix casting of overlapped to writebuf

Change-Id: Ib39ca14eda1a4763a1d080fa74294f5dc0dfc323
Reviewed-on: http://review.couchbase.org/29281
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>