Issue252

I tried to fix issue #252.
And, I fixed the way of getting event data.

Please check the code. And, if there is some problem, let me know.
I moved it at bd5f97c revision.

But I think it is unnecessary checking the value of first event ID.
Because, when Zabbix doesn't have events, [ArmZabbixAPI.cc L185-188](https://github.com/project-hatohol/hatohol/blob/bd5f97c40f6d8421f1e34bb15f74e8e5e8b9b550/server/src/ArmZabbixAPI.cc#L185-188) doesn't work.