No PB listeners leads to repeated log messages concerning a failed stat calculation

When no PB Listeners are configured, repeated log messages concerning a failed stat calculation are output:

```
23:09:59.144 [warning] Failed to calculate stat {riak_api,pbc_connects,active} with exit:{noproc,{gen_server,call,[riak_api_pb_sup,count_children,infinity]}}
23:10:00.225 [warning] Failed to calculate stat {riak_api,pbc_connects,active} with exit:{noproc,{gen_server,call,[riak_api_pb_sup,count_children,infinity]}}
23:10:01.300 [warning] Failed to calculate stat {riak_api,pbc_connects,active} with exit:{noproc,{gen_server,call,[riak_api_pb_sup,count_children,infinity]}}
23:10:02.373 [warning] Failed to calculate stat {riak_api,pbc_connects,active} with exit:{noproc,{gen_server,call,[riak_api_pb_sup,count_children,infinity]}}
23:10:03.251 [warning] Failed to calculate stat {riak_api,pbc_connects,active} with exit:{noproc,{gen_server,call,[riak_api_pb_sup,count_children,infinity]}}
23:10:03.448 [warning] Failed to calculate stat {riak_api,pbc_connects,active} with exit:{noproc,{gen_server,call,[riak_api_pb_sup,count_children,infinity]}}
```