Fix ginetaddress.c compile on Solaris

Fix ginetaddress.c compile on Solaris

In glibc, IN6_IS_ADDR_UNSPECIFIED() et al. cast their argument to a
uint32_t*, so it doesn't matter whether you pass them the in6_addr
itself (which is what you're supposed to do) or one of its union
members (which is what we were actually doing). Solaris's macro
accesses the in6_addr fields directly though, and so only works if you
pass the actual in6_addr. #580194.