rss: protect against nil field

This happens when the filter tries to wrap a non-existing field, and in
custom filters it's probably a sign that the wrong field are being
accessed. It would probably be nicer to raise a warning about this, but
I'm too lazy to add that now.

data/rbot/plugins/rss.rb

@@ -392,7 +392,7 @@ def make_stream(line1, line2, s, h={})
           # do nothing
         else
           warning "ignoring #{v.inspect} wrapping of unknown class"
-        end
+        end unless ss[nk].nil?
       else
         subs[k] = v
       end