java: discard the IPv6 zone id when parsing addresses

Zone IDs are used in link-local addresses to specify the interface name or
number (RFC 4007). The InetSocketAddress class includes a zone id in the output
of getHostString() for link-local addresses. This change ignores the zone id
instead of raising an exception.
IPv6Address: remove redundant null assignment
Merge into master from pull request #309:
java: discard the IPv6 zone id when parsing addresses (https://github.com/floodlight/loxigen/pull/309)