Fix off-by-one error when using the len(prefix) to do slicing.

This fixes errors such as:
```
Could not find a valid timestamp from the index: logstash-2013.09.08
Could not find a valid timestamp from the index: logstash-2013.09.09
Could not find a valid timestamp from the index: logstash-2013.09.10
Could not find a valid timestamp from the index: logstash-2013.09.11
Could not find a valid timestamp from the index: logstash-2013.09.12
```
... even though the prefix and separator are correct.