Fix broken pool drainer after the upgrade.

See 6cd5cc375a/activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb (L467)

We think there is a leak at the moment because a discarded pool
belonging to the parent process shouldn't be present in the
forked process.

lib/freedom_patches/pool_drainer.rb

@@ -26,13 +26,15 @@ class ActiveRecord::ConnectionAdapters::ConnectionPool
   # if idle_time is specified only connections idle for N seconds will be drained
   def drain(idle_time = nil, max_age = nil)
     synchronize do
-      @available.clear
-      @connections.delete_if do |conn|
-        try_drain?(conn, idle_time, max_age)
-      end
+      if @available && @connections
+        @available.clear
+        @connections.delete_if do |conn|
+          try_drain?(conn, idle_time, max_age)
+        end
 
-      @connections.each do |conn|
-        @available.add conn if !conn.in_use?
+        @connections.each do |conn|
+          @available.add conn if !conn.in_use?
+        end
       end
     end