mach: Fix bogus negative return

One can be very unlucky to call time_now first just before a second switch,
and mach_msg sleep just a bit more enough for the second time_now call to
count one second too many (or even more if scheduling is really unlucky).

So we have to protect against returning a bogus negative value in such case.

sysdeps/mach/sleep.c

@@ -43,6 +43,9 @@ __sleep (unsigned int seconds)
   after = time_now ();
   __mach_port_destroy (__mach_task_self (), recv);
 
+  if (after - before > seconds)
+    return 0;
+
   return seconds - (after - before);
 }
 weak_alias (__sleep, sleep)