Don't raise alerts for DNS records that point to 0.0.0.0

[anjackson]

When scanning previously-known domains, Heritrix really does not like DNS records that point to 0.0.0.0 and raises alerts like this:

SEVERE: Problem java.lang.IllegalStateException: got suspicious value 0.0.0.0 for www.absolute-lofts.co.uk occurred when trying to process 'http://www.absolute-lofts.co.uk/' at step ABOUT_TO_BEGIN_PROCESSOR in

java.lang.IllegalStateException: got suspicious value 0.0.0.0 for www.absolute-lofts.co.uk
        at org.archive.modules.fetcher.DefaultServerCache.getHostFor(DefaultServerCache.java:112)
        at org.archive.modules.net.ServerCache.getHostFor(ServerCache.java:64)
        at org.archive.crawler.prefetch.QuotaEnforcer.innerProcessResult(QuotaEnforcer.java:392)
        at org.archive.modules.Processor.process(Processor.java:142)
        at org.archive.modules.ProcessorChain.process(ProcessorChain.java:131)
        at org.archive.crawler.framework.ToeThread.run(ToeThread.java:147)

This clogs up the alert logs. Unfortunately it seems like this is a common practice these days, so perhaps Heritrix should just quietly ignore this condition rather than raise an IllegalStateException.