don't fail on undefined values in last stats sample.MB-3867
don't fail on undefined values in last stats sample.MB-3867

For stats counters when previous value is absent stats_collector puts
undefined to this stats. So we need to handle it. Rest of
menelaus_stats handle it since early days.

This fixes part of MB-3867.

Change-Id: Ife7b3610e38c334b033f0830e6ec2725ab5fae74
Reviewed-on: http://review.membase.org/6367
Tested-by: Aliaksey Kandratsenka <340b8e09ca65cd3fc686427fcfed17e87eaf61e2@gmail.com>
Reviewed-by: Steve Yen <ef850f73f7832052d3c6d9b5ddd3a1716d9a57f1@gmail.com>

pavel-blagodov/ns_server,mschoch/ns_server,t3rm1n4l/ns_server,nimishzynga/ns_server,t3rm1n4l/ns_server,ceejatec/ns_server,ceejatec/ns_server,ceejatec/ns_server,membase/ns_server,ceejatec/ns_server,membase/ns_server,mschoch/ns_server,t3rm1n4l/ns_server,mschoch/ns_server,daverigby/ns_server,membase/ns_server,mschoch/ns_server,membase/ns_server,nimishzynga/ns_server,daverigby/ns_server,vzasade/ns_server,nimishzynga/ns_server,pavel-blagodov/ns_server,mschoch/ns_server,vzasade/ns_server,ceejatec/ns_server,nimishzynga/ns_server,membase/ns_server,nimishzynga/ns_server,vzasade/ns_server,daverigby/ns_server,nimishzynga/ns_server,daverigby/ns_server,pavel-blagodov/ns_server,membase/ns_server,vzasade/ns_server,t3rm1n4l/ns_server,ceejatec/ns_server,mschoch/ns_server,t3rm1n4l/ns_server,daverigby/ns_server,pavel-blagodov/ns_server