Fixed string parsing logic

Fixed this error which used to come up previously :-
Traceback (most recent call last):
  File "github-backup.py", line 55, in <module>
    main()
  File "github-backup.py", line 22, in main
    process_repo(repo, args)
  File "github-backup.py", line 45, in process_repo
    print("Processing repo: {}".format(repo.full_name))
ValueError: zero length field name in format