hurdsock: reject negative domains

Reject 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.

2011-11-21  Pino Toscano  <toscano.pino@tiscali.it>

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