Ignore nonexistent "source" fields

[philhagen]

Version 3.1.3 fails if a non-existent field is provided in the source array. The plugin should silently skip any nonexistent field specified here so the configuration can provide a list of potential fields and tag accordingly.

At present, a Invalid IP address, skipping {:address=>"%{field_name}" log is created.

[philhagen]

I suspect this might be implemented by ignoring blank/nonexistent value for a at this point:

logstash-filter-cidr/lib/logstash/filters/cidr.rb

Line 126 in d963e92

IPAddr.new(event.sprintf(a))

However, my ruby skills are thin to nonexistent so I am hesitant to suggest a specific code change.