Errors in the python netstats module

I had a lot of the following errors in the syslog of idle nodes :
```
[PYTHON] Can't call the metric handler function for [tcp_retrans_percentage] in the python module [netstats].#012
[PYTHON] Can't call the metric handler function for [tcpext_tcploss_percentage] in the python module [netstats].#012
```

I tracked down the error being a python ZeroDivisionError exception when the values of 'insegs' and 'outsegs' didn't change in the default 15 seconds interval between metric collection.