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/screen.c b/src/screen.c
index 3bf15d6..c5c6a7a 100644
--- a/src/screen.c
+++ b/src/screen.c
@@ -2439,7 +2439,7 @@
     FOR_ALL_TAB_WINDOWS(tp, wp)
 	win_free_lsize(wp);
     for (int i = 0; i < AUCMD_WIN_COUNT; ++i)
-	if (aucmd_win[i].auc_win_used)
+	if (aucmd_win[i].auc_win != NULL)
 	    win_free_lsize(aucmd_win[i].auc_win);
 #ifdef FEAT_PROP_POPUP
     // global popup windows
@@ -2484,7 +2484,7 @@
 	}
     }
     for (int i = 0; i < AUCMD_WIN_COUNT; ++i)
-	if (aucmd_win[i].auc_win_used
+	if (aucmd_win[i].auc_win != NULL
 		&& aucmd_win[i].auc_win->w_lines == NULL
 		&& win_alloc_lines(aucmd_win[i].auc_win) == FAIL)
 	{