FIX: Render a 404 error on a bad redirect in list controller

When bad data is provided in the URI for redirecting to a category,
Rails raises an `ActionController::Redirecting::UnsafeRedirectError`
error, leading to a 500 error.

This patch catches the exception to render a 404 instead.

app/controllers/list_controller.rb

@@ -49,6 +49,10 @@ class ListController < ApplicationController
                   :filter,
                 ].flatten
 
+  rescue_from ActionController::Redirecting::UnsafeRedirectError do
+    raise Discourse::NotFound
+  end
+
   # Create our filters
   Discourse.filters.each do |filter|
     define_method(filter) do |options = nil|