fixed bug when uploading directly

Uploading files less than 5MB when no s3 key exists causing the following error:

> Traceback (most recent call last):
>  File "/usr/local/sbin/s3-mp-upload.py", line 155, in <module>
>    main(**arg_dict)
>  File "/usr/local/sbin/s3-mp-upload.py", line 102, in main
>    key.set_contents_from_file(src)
> AttributeError: 'NoneType' object has no attribute 'set_contents_from_file'

This commit fixes it.