idle-monitor: avoid XSyncBadAlarm X error

If we fail to find the IDLETIME counter, then the alarm variable will be
uninitialised.  Most code paths are careful to check this before
submitting XSync calls, but there is one check missing.

https://bugzilla.gnome.org/show_bug.cgi?id=724364

src/core/meta-idle-monitor.c

@@ -499,7 +499,7 @@ make_watch (MetaIdleMonitor           *monitor,
       if (meta_idle_monitor_get_idletime (monitor) > (gint64)timeout_msec)
         watch->idle_source_id = g_idle_add (fire_watch_idle, watch);
     }
-  else
+  else if (monitor->user_active_alarm != None)
     {
       watch->xalarm = monitor->user_active_alarm;