Deal gracefully with non-URL input

This should close issue https://github.com/HyShai/url-cleaner/issues/3

Test cases passed:
print(url_lengthen(None))
print(url_lengthen(0))
print(url_lengthen(-6))
print(url_lengthen('None'))
print(url_lengthen('zippy://pinhead.com'))
print(url_lengthen('http://www.apple.com'))