Fix checking empty directory

Fix error like
```
Checking daily backup of tbbc-prod1:/var ...
Traceback (most recent call last):
  File "/home/phunehehe/check-amanda/check-amanda.py", line 152, in <module>
    main()
  File "/home/phunehehe/check-amanda/check-amanda.py", line 143, in main
    random_file = get_file_list(config, hostname, disk)
  File "/home/phunehehe/check-amanda/check-amanda.py", line 79, in get_file_list
    current_path = choice(paths)
  File "/usr/lib/python2.7/random.py", line 274, in choice
    return seq[int(self.random() * len(seq))]  # raises IndexError if seq is empty
IndexError: list index out of range
```
happens when the selected directory is empty