posts: fix error on post index page when search count times out in safe mode

Fix a nil deference error on the post index page. This happened when
performing a single tag search in safe mode and calculating the number
of search results timed out.

app/logical/post_query.rb

@@ -201,6 +201,7 @@ class PostQuery
   end
 
   concerning :CountMethods do
+    # @return [Integer, nil] The number of posts returned by the search, or nil on timeout.
     def post_count
       @post_count ||= fast_count
     end