stubdom: support select with NULL arguments

diff -r 73ec21ad5dc7 extras/mini-os/lib/sys.c
@@ -820,9 +820,19 @@
     add_waiter(w4, xenbus_watch_queue);
     add_waiter(w5, kbdfront_queue);
 
-    myread = *readfds;
-    mywrite = *writefds;
-    myexcept = *exceptfds;
+    if (readfds)
+        myread = *readfds;
+    else
+        FD_ZERO(&myread);
+    if (writefds)
+        mywrite = *writefds;
+    else
+        FD_ZERO(&mywrite);
+    if (exceptfds)
+        myexcept = *exceptfds;
+    else
+        FD_ZERO(&myexcept);
+
     DEBUG("polling ");
     dump_set(nfds, &myread, &mywrite, &myexcept, timeout);
     DEBUG("\n");

@@ -861,9 +871,19 @@
 	thread->wakeup_time = stop;
     schedule();
 
-    myread = *readfds;
-    mywrite = *writefds;
-    myexcept = *exceptfds;
+    if (readfds)
+        myread = *readfds;
+    else
+        FD_ZERO(&myread);
+    if (writefds)
+        mywrite = *writefds;
+    else
+        FD_ZERO(&mywrite);
+    if (exceptfds)
+        myexcept = *exceptfds;
+    else
+        FD_ZERO(&myexcept);
+
     n = select_poll(nfds, &myread, &mywrite, &myexcept);
 
     if (n) {