votes: fix division by zero in upvote ratio.

Fix a division by zero when calculating the upvote ratio in the votes tooltip.

This could happen if the post had votes according to the post_votes
table, but didn't have any votes according to the up_score and
down_score. This should never happen, yet it did happen for post 2248039
and post 2959269.

app/components/post_votes_tooltip_component.rb

@@ -19,9 +19,13 @@ class PostVotesTooltipComponent < ApplicationComponent
     vote.is_positive? ? upvote_icon : downvote_icon
   end
 
+  def vote_count
+    post.up_score + post.down_score.abs
+  end
+
   def upvote_ratio
-    return nil if votes.length == 0
-    sprintf("(%.1f%%)", 100.0 * post.up_score / (post.up_score + post.down_score.abs))
+    return nil if vote_count == 0
+    sprintf("(%.1f%%)", 100.0 * post.up_score / vote_count)
   end
 
   def voter_name(vote)