hurd: recvfrom(): take into account null address ports

Some kinds of sockets may return a null address port when calling the
`socket_recv' RPC, so avoid using it in case an address argument buffer is
asked to be filled.  In such case, set the length of that address buffer to
zero.

2011-11-26  Pino Toscano  <toscano.pino@tiscali.it>

	* sysdeps/mach/hurd/recvfrom.c (__recvfrom): Check also for a null
	address port.  Set `addr_len' to 0 when not filling `addrarg'.
--- a/sysdeps/mach/hurd/recvfrom.c
+++ b/sysdeps/mach/hurd/recvfrom.c
@@ -55,7 +55,7 @@
     return __hurd_sockfail (fd, flags, err);
 
   /* Get address data for the returned address port if requested.  */
-  if (addr != NULL)
+  if (addr != NULL && addrport != MACH_PORT_NULL)
     {
       char *buf = (char *) addr;
       mach_msg_type_number_t buflen = *addr_len;
@@ -89,6 +89,8 @@
       if (buflen > 0)
 	addr->sa_family = type;
     }
+  else if (addr_len != NULL)
+    *addr_len = 0;
 
   __mach_port_deallocate (__mach_task_self (), addrport);