Reset primary_server when remove_server

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@65214 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/drb/drb.rb

@@ -1866,6 +1866,11 @@ module DRb
   # Removes +server+ from the list of registered servers.
   def remove_server(server)
     @server.delete(server.uri)
+    mutex.synchronize do
+      if @primary_server == server
+        @primary_server = nil
+      end
+    end
   end
   module_function :remove_server