mno-async_writes/client: Minor modifications in TestRPCServer.

- Disabled check for TestRPCServer implementations with zero registered operations.
- Stop() can be called after a failed Start() without null pointer errors now.

git-svn-id: https://xtreemfs.googlecode.com/svn/branches/mno-async_writes@2982 f8528748-c2c0-11dd-ba5c-d3b7c2b3b534