os: Fix a type error in the IPv6 XDMCP code

Building with strict-aliasing rightly chirps here:

../os/xdmcp.c: In function ‘XdmcpRegisterConnection’:
../os/xdmcp.c:489:31: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
                     &((struct sockaddr_in6 *) &address)->sin6_addr.s6_addr[12];
                               ^~~~~~~~~~~~

We have "const char *address", so &address here is a char ** (i.e., it
points to the slot on the stack containing the pointer to the character
array passed in as an argument). Casting that to a struct sockaddr_in6 *
is wrong, because it means that area of the stack will be reinterpreted
as a struct sockaddr_in6.

Instead, cast address, not &address.

Signed-off-by: Adam Jackson <ajax@redhat.com>