Deal with cases where the stats aren't available

Right now it throws an exception:

```
NoMethodError: undefined method `>=' for nil:NilClass 
/usr/lib/ruby/gems/1.8/gems/scout-5.6.9/lib/scout/plugin.rb:183:in `counter' 
Mapper Queue (Kestrel):24:in `build_report' 
/usr/lib/ruby/gems/1.8/gems/scout-5.6.9/lib/scout/plugin.rb:220:in `run' 
/usr/lib/ruby/gems/1.8/gems/scout-5.6.9/lib/scout/server.rb:398:in `process_plugin' 
/usr/lib/ruby/1.8/timeout.rb:67:in `timeout' 
```