MB-10259 Check for NULL object when parsing vbstat json doc

Log a warning if cJSON returns NULL when parsing vbstat json doc

Change-Id: I6a7276ba3a5aaa49547758386f78db5a09990539
Reviewed-on: http://review.couchbase.org/34004
Reviewed-by: David Liao <david.liao@couchbase.com>
Tested-by: David Liao <david.liao@couchbase.com>