uninitialized value when warn/crit thresholds not specified

On fully updated Ubuntu 14.04 LTS systems the script will spit these warnings when not specifying -w and/or -c:

```
Use of uninitialized value in split at ./check_json.pl line 142.
Use of uninitialized value in split at ./check_json.pl line 143.
```

When -w is given the former goes away, when -c is given the latter goes away.
Seems like a minor, cosmetic issue.
