MB-11026 Don't allow the underflow on the dirty queue size stat.

We will find the root cause of the underflow issue separately
soon.

Change-Id: Id5459fe3ac14f850159e9321791137860f265f62
Reviewed-on: http://review.couchbase.org/37153
Reviewed-by: abhinav dangeti <abhinav@couchbase.com>
Tested-by: abhinav dangeti <abhinav@couchbase.com>