xdm crash on login from IPv6 Xserver

[Marc Olzheim]

xdm crashes in auth.c:1218, because the result of ConvertAddr() of -1 is
not checked, while leaving addr at NULL...

Adding a if (0 < family) {} block around it, prevents it from crashing,
but still does not allow for IPv6 xdmcp logins.

Connecting to a running Xserver (port 6000) over IPv6 does work though.
It's not possible to connect to an IPv6 xdm with X -query, because it
cannot resolve / parse / whatever the hostname... We made the IPv6 xdmcp
connection with a sparc running Solaris 10 beta.

This is with sysctl net.inet6.ip6.v6only = 1

Zlo