Break out of the loop after shutdown in DRbServer

* Patch by @seki:
  https://gist.github.com/seki/ae8bef20238c37c94a91b5461c4bfbdd

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@61342 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/drb/drb.rb

@@ -1678,7 +1678,10 @@ module DRb
             error_print(e) if verbose
           ensure
             client.close unless succ
-            shutdown if Thread.current['DRb']['stop_service']
+            if Thread.current['DRb']['stop_service']
+              shutdown
+              break
+            end
             break unless succ
           end
         end