Address net-dns additional records empty array

Conversion between packet formats can create empty additional
answers fields, which net-dns cannot handle.

Update net-dns' packet parser to be able to deal with empty arrays
such that it doesn't try to call :data on a nil, which is the only
element of an empty array.

Props to mubix for identifying this issues.

lib/net/dns/packet.rb

@@ -184,6 +184,7 @@ module Net # :nodoc:
           nscount += 1
         end
         @additional.each do |rr|
+          next if rr.nil?
           data += rr.data#(data.length)
           arcount += 1
         end