Check queue length when it is safe to do
Check queue length when it is safe to do

'queue' variable must be used only when try clause does not raise an
exception, otherwise we die with:

Traceback (most recent call last):
  File "/foo.../bin/haproxystats-pull", line 10, in <module>
    sys.exit(main())
  File "/foo.../haproxystats/pull.py", line 416, in main
    supervisor(loop, config, executor)
  File "/foo.../haproxystats/pull.py", line 239, in supervisor
    if len(queue) >= config.getint('pull', 'queue-size'):
UnboundLocalError: local variable 'queue' referenced before assignment

unixsurfer/haproxystats,unixsurfer/haproxystats,unixsurfer/haproxystats