[fluentd] Added error handling for `strconv.Atoi(f.Port)`.
[fluentd] Added error handling for `strconv.Atoi(f.Port)`.

- I was ignoring the error.

mackerelio/mackerel-agent-plugins,mackerelio/mackerel-agent-plugins,mackerelio/mackerel-agent-plugins,mackerelio/mackerel-agent-plugins