fixes #10 by catching exception

This fixes #10 but it still generates a warning:

```
record_reformer: NoMethodError undefined method `[]' for nil:NilClass /Users/marshall/code/random/fluent-plugin-record-reformer/lib/fluent/plugin/out_record_reformer.rb:190:in `block in expand'
```

I'm not sure what the best way to handle this is, this is an improvement because it does not stop it from processing the message, which I think is better than the current behavior in which it will stop processing. 

Since we have several different message formats we do not want to stop processing. 
thanks for the clarification, what's the best strategy then?

i was thinking we could return `nil` [here](https://github.com/sonots/fluent-plugin-record-reformer/blob/master/lib/fluent/plugin/out_record_reformer.rb#L109). this will then output something like `(nil, 1412779402, nil)`

i'm trying to understand the best behavior. do we act like the message never was processed by this plugin or do we warn and not return the message (stopping processing)?