patch 9.0.0967: leaking memory from autocmd windows

Problem:    Leaking memory from autocmd windows.
Solution:   Free window when auc_win is not NULL.
diff --git a/src/window.c b/src/window.c
index 24f25d6..9a49cad 100644
--- a/src/window.c
+++ b/src/window.c
@@ -3293,10 +3293,10 @@
 	tabpage_close(TRUE);
 
     for (int i = 0; i < AUCMD_WIN_COUNT; ++i)
-	if (aucmd_win[i].auc_win_used)
+	if (aucmd_win[i].auc_win != NULL)
 	{
 	    (void)win_free_mem(aucmd_win[i].auc_win, &dummy, NULL);
-	    aucmd_win[i].auc_win_used = FALSE;
+	    aucmd_win[i].auc_win = NULL;
 	}
 
     while (firstwin != NULL)