xcb_auth: Fix memory leak in _xcb_get_auth_info.

If the initial get_peer_sock_name(getpeername ...) succeeds, the
pointer to allocated memory is overwritten by the later call to
get_peer_sock_name(getsockname ...).  Fix that up by freeing
the allocated memory before overwriting the pointer.

Signed-off-by: Nick Bowler <nbowler@draconx.ca>
Signed-off-by: Julien Danjou <julien@danjou.info>