When there is only one slice and one scanner, a bwauth runs a second empty slice
When I run a single bandwidth scanner on less than 50 nodes, configured with:
```
start_pct = 0
stop_pct = 100
nodes_per_slice = 50
```
I get the following log message:
```
NOTICE[Mon Oct 31 04:31:46 2016]:Starting slice for percentiles 100-200
NOTICE[Mon Oct 31 04:31:47 2016]:Only 0 exits remain in slice 100-200 after restrictions. ...
NOTICE[Mon Oct 31 04:31:47 2016]:No routers left after restrictions applied: NodeRestrictionList(['UnmeasuredPercentileRestriction(100,200)', 'OrNodeRestriction(["FlagsRestriction([\'BadExit\'],[])", \'ConserveExitsRestriction()\'])', "FlagsRestriction(['Running'],[])"])
```
as well as a successful message for the 0-100 slice.
It seems likely that there is an off by one error, either in the handling of start_pct & stop_pct, or in the handling of nodes_per_slice.
issue