fixup: Check buffer for retry_packet

Change-Id: Iba09f499028a8e458633864fdef57e0c713f7619
Reviewed-on: http://review.couchbase.org/27777
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>