[client][utils] Check existence of hostId at getHostName()

An error is occured on my broken environment (old data are remained
and cannot connect to a host):

  [http://localhost:8000/static/js/utils.js:165]
  TypeError: server.hosts[hostId] is undefined

Although it's not happend on usual cases (maybe), we should check
existence of the element before accessing to it.
Merge pull request #185 from project-hatohol/check-existence-of-hostid

[client][utils] Check existence of hostId at getHostName()