Check if pfringclusterid is zero

[JustinAzoff]

Until https://bro-tracker.atlassian.net/browse/BIT-1864 is fixed, it will be a common problem for people trying to use the native pf_ring plugin.

if pfring_configured is True, pfringclusterid should not be zero.. this should potentially just be checked inside the plugin... The current behavior makes almost no sense.