convertible: abort with an error if Page/Post#data isn't a hash.

when the default_proc was being assigned, it failed if it wasn't a Hash. We
expect data to be a Hash everywhere, so let's freak out if it isn't after
reading and applying the fallback.

Fixes #3643.

lib/jekyll/convertible.rb

@@ -56,6 +56,10 @@ module Jekyll
       end
 
       self.data ||= {}
+
+      unless self.data.is_a?(Hash)
+        Jekyll.logger.abort_with "Fatal:", "Invalid YAML front matter in #{File.join(base, name)}"
+      end
     end
 
     # Transform the contents based on the content type.