URLParser fails if URL contains empty port

[sebastian-nagel]

If the port in a URL is empty (e.g., in http://example.com:/) the URLParser throws an exception:

java.net.URISyntaxException: bad port : http://fc-zenit.ru:/news/turnir-pamyati-sadyrina/2010/12/17/odin-iz-chetyrnadcati/
     at org.archive.url.URLParser.parse(URLParser.java:253)
     at org.archive.url.WaybackURLKeyMaker.makeKey(WaybackURLKeyMaker.java:60)

Cf. NUTCH-2337