cbstats -a does not work when there is a memcached bucket

When cbstats use -a, it get the bucket list and steps through it. If the stats are not available for the bucket (i.e memcached bucket) it will exit without collect the stats from the other buckets.

This line needs to be changed to a continue instead of a exit:
http://src.couchbase.org/source/xref/4.6.1/ep-engine/management/cbstats#69

Marking this as Critical as it affects cbcollect