Allow to stop by push(nil) for test

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@65243 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/drb/extservm.rb

@@ -61,8 +61,7 @@ module DRb
     private
     def invoke_thread
       Thread.new do
-        while true
-          name = @queue.pop
+        while name = @queue.pop
           invoke_service_command(name, @@command[name])
         end
       end