fix: download small zip error

[horw]

When you tried to download a small zip file, you encountered the following error. This PR fix it.

Traceback (most recent call last):
  File "/home/horw/PycharmProjects/unzip-http/unzip-http", line 122, in <module>
    main(args)
  File "/home/horw/PycharmProjects/unzip-http/unzip-http", line 103, in main
    for f in rzf.infolist():
  File "/home/horw/PycharmProjects/unzip-http/unzip_http.py", line 119, in infolist
    return list(self.infoiter())
  File "/home/horw/PycharmProjects/unzip-http/unzip_http.py", line 147, in infoiter
    error('cannot find central directory')
  File "/home/horw/PycharmProjects/unzip-http/unzip_http.py", line 35, in error
    raise Exception(s)
Exception: cannot find central directory

Test file:
hello.zip

[saulpw]

Looks good to me, @anjakefala can you verify?

[anjakefala]

@horw Works like a charm!