Fix error raised when ext is None, since python cannot evaulate len(None)

[jonas-t-s]

Hello there

I found out, that in the code ext max be None, which is foreseen by the next if statement. But the statement len(ext) is evaluated before that if statement. This raises an error which aborts Moodle-DL.

I fix that in this pull request with checking if ext is not None and then evaluete the lenght of it. Otherwise I set the length to 0(which is not used but for completness I added it) .

[aaannndddyyy]

Thanks. It worked for me and fixed the issue (issue #207 )where I could not download anything with moodle-dl