Fix bug I introduced by changes in recent commits.

Relevant to #38.

I introduced a bug by creating a condition where the `suffix_list_source`
variable would never get assigned. Now in that case you should get an empty TLD
list, and an error-level log message, instead of a crash.

Traceback something like:

    tlds = get_tlds_from_suffix_list_source(suffix_list_source)
    # UnboundLocalError: local variable 'suffix_list_source' referenced before assignment