FIX: don't crash out searching for parent in oneboxer

lib/oneboxer.rb

@@ -82,13 +82,14 @@ module Oneboxer
     changed = false
 
     Oneboxer.each_onebox_link(doc) do |url, element|
-      onebox, preview = yield(url,element)
+      onebox, _preview = yield(url,element)
       if onebox
         parsed_onebox = Nokogiri::HTML::fragment(onebox)
         next unless parsed_onebox.children.count > 0
 
         # special logic to strip empty p elements
         if  element.parent &&
+            element.parent.node_name &&
             element.parent.node_name.downcase == "p" &&
             element.parent.children.count == 1
           element = element.parent