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

app/logical/post_sets/post.rb

@@ -35,7 +35,8 @@ module PostSets
 
     # The description of the page for the <meta name="description"> tag.
     def meta_description
-      if post_query.is_simple_tag?
+      # XXX post_count may be nil if the search times out because of safe mode
+      if normalized_query.is_simple_tag? && post_count.present?
         humanized_count = ApplicationController.helpers.humanized_number(post_count, million: " million", thousand: " thousand")
         humanized_count = "over #{humanized_count}" if post_count >= 1_000
 
@@ -108,6 +109,7 @@ module PostSets
       @posts ||= normalized_query.paginated_posts(page, includes: includes, count: post_count, search_count: !post_count.nil?, limit: per_page, max_limit: max_per_page).load
     end
 
+    # @return [Integer, nil] The number of posts returned by the search, or nil if unknown.
     def post_count
       normalized_query.post_count
     end

test/functional/posts_controller_test.rb

@@ -234,6 +234,13 @@ class PostsControllerTest < ActionDispatch::IntegrationTest
           assert_response 410
           assert_select "h1", "Search Error"
         end
+
+        should "render if the search count times out" do
+          PostQuery.any_instance.stubs(:exact_count).returns(nil)
+          get posts_path, params: { tags: "1girl", safe_mode: "true" }
+
+          assert_response :success
+        end
       end
 
       context "with a pool: search" do