CCBC-394: Check ringbuffer on server_start_packet_ex

Change-Id: If979226c3cdd2997ef56609ef7976968fef45bc8
Reviewed-on: http://review.couchbase.org/36525
Reviewed-by: Yury Alioshinov <haster2010@gmail.com>
Tested-by: Yury Alioshinov <haster2010@gmail.com>
Reviewed-by: Brett Lawson <brett19@gmail.com>