Fix off by 1 error in stats.py resulting in additional request always being off

[aldenpeterson-wf]

Address #399, which is the same issue we just hit in our usage.

The counter is incremented in log but it was being checked for the end condition before that point was hit.
This meant that if you ran 3 requests using the command line mode:

locust --no-web -c 1 -r 1 -n 3 -f tornado_nats_locust.py

you would end with Locust actually executing 4 tasks.

FYI @cgoldberg @heyman @seangerhardt-wf

[heyman]

Thanks!