Fix authentication on hpux and Hurd

libxcb's 010e5661 (Fix XDM-AUTHORIZATION-1 (bug #14202)) mistakenly
inverted a few lines of code, making local socket authentication fail on
hpux and Hurd: when getpeername fails, sockname needs to be initialized
by getsockname before its address family can be checked.

Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Signed-off-by: Julien Danjou <julien@danjou.info>