Fix #3991: Loading post #1604318 is broken.

Fix exception in Pool#previous_post_id when the post id is not contained
in the pool.

This can happen when a post's pool_string lists the post as being in a
certain pool, but the post is not included in the pool's post_ids. Such
pool_string / post_ids inconsistencies exist because of past bugs.

app/models/pool.rb

@@ -268,19 +268,25 @@ class Pool < ApplicationRecord
   end
 
   def first_post?(post_id)
-    page_number(post_id) == 1
+    post_id == post_ids.first
+  end
+
+  def last_post?(post_id)
+    post_id == post_ids.last
   end
 
   # XXX finds wrong post when the pool contains multiple copies of the same post (#2042).
   def previous_post_id(post_id)
+    return nil if first_post?(post_id) || !contains?(post_id)
+
     n = post_ids.index(post_id) - 1
-    return nil if n < 0
     post_ids[n]
   end
 
   def next_post_id(post_id)
+    return nil if last_post?(post_id) || !contains?(post_id)
+
     n = post_ids.index(post_id) + 1
-    return nil if n >= post_ids.size
     post_ids[n]
   end