Fix error when resuming archive with no completed parts

Fixes this error caused when using --resume and an archive that has 0 completed parts.

```
Traceback (most recent call last):
  File "/usr/local/bin/glacier-cmd", line 9, in <module>
    load_entry_point('glacier==0.2dev', 'console_scripts', 'glacier-cmd')()
  File "/usr/local/lib/python2.7/dist-packages/glacier-0.2dev-py2.7.egg/glacier/glacier.py", line 929, in main
    args.func(args)
  File "/usr/local/lib/python2.7/dist-packages/glacier-0.2dev-py2.7.egg/glacier/glacier.py", line 156, in wrapper
    return fn(*args, **kwargs)
  File "/usr/local/lib/python2.7/dist-packages/glacier-0.2dev-py2.7.egg/glacier/glacier.py", line 309, in upload
    args.name, args.partsize, args.uploadid, args.resume)
  File "/usr/local/lib/python2.7/dist-packages/glacier-0.2dev-py2.7.egg/glacier/GlacierWrapper.py", line 65, in wrapper
    ret = fn(*args, **kwargs)
  File "/usr/local/lib/python2.7/dist-packages/glacier-0.2dev-py2.7.egg/glacier/GlacierWrapper.py", line 232, in glacier_connect_wrap
    return func(*args, **kwargs)
  File "/usr/local/lib/python2.7/dist-packages/glacier-0.2dev-py2.7.egg/glacier/GlacierWrapper.py", line 65, in wrapper
    ret = fn(*args, **kwargs)
  File "/usr/local/lib/python2.7/dist-packages/glacier-0.2dev-py2.7.egg/glacier/GlacierWrapper.py", line 253, in sdb_connect_wrap
    return func(*args, **kwargs)
  File "/usr/local/lib/python2.7/dist-packages/glacier-0.2dev-py2.7.egg/glacier/GlacierWrapper.py", line 65, in wrapper
    ret = fn(*args, **kwargs)
  File "/usr/local/lib/python2.7/dist-packages/glacier-0.2dev-py2.7.egg/glacier/GlacierWrapper.py", line 1105, in upload
    writer.uploaded_size = stop
UnboundLocalError: local variable 'stop' referenced before assignment
```