Update poster.py

if run the script like as "python poster.py --describe_only" will catch exception:
```
Traceback (most recent call last):
  File "worker.py", line 184, in <module>
    total_records = sum_posts(threads)
NameError: name 'threads' is not defined
```
Merge pull request #2 from jumping/patch-1

Fixed NameError when running Poster with ```--describe_only```