Refuse negative socket domains right away; otherwise, it is possible to read
and set out-of-bounds locations of the `servers' array (returning the values
at those invalid memory locations), and even try to deallocate ports with
random values if the `dead' parameter is different than zero.

* hurd/hurdsock.c (_hurd_socket_server): Check for negative domains.
--- a/hurd/hurdsock.c
+++ b/hurd/hurdsock.c
@@ -47,6 +47,12 @@
 {
   socket_t server;
 
+  if (domain < 0)
+    {
+      errno = EAFNOSUPPORT;
+      return MACH_PORT_NULL;
+    }
+
   HURD_CRITICAL_BEGIN;
   __mutex_lock (&lock);