dix: NULL out WindowTable after freeing all the windows.

CloseDownDevices() tries to send PresenceNotify events. If the windows are
already freed, then we are accessing dangling pointers.
(cherry picked from commit aec485f2dc)

dix/main.c

@@ -458,7 +458,10 @@ main(int argc, char *argv[], char *envp[])
 #endif
 
         config_fini();
+
+        memset(WindowTable, 0, MAXSCREENS * sizeof(WindowPtr));
 	CloseDownDevices();
+
 	for (i = screenInfo.numScreens - 1; i >= 0; i--)
 	{
 	    FreeScratchPixmapsForScreen(i);