Don't pass random value to XdmcpFill

commit e533f4b7
(combined with commit f7a7158e
and commit 86ac1662)
changed the length passed to XdmcpFill to use gdm_sockaddr_len,
macro.  This macro is being called on an uninitialized structure,
so it returns the wrong value.

I think commit e533f4b7 was a bit
too liberal in its gdm_sockaddr_len changes.  While I can believe
some Solaris and FreeBSD networking calls require the exact socket
address structure length to be passed in, I doubt the call used
by XdmcpFill (recvfrom) is one of them. It would be impossible for
recvfrom to work for connection of different address families
if that were true.

This commit just reverts gdm_sockaddr_len-ification of those the
two places where XdmcpFill is called.