vim-patch:9.0.2106: [security]: Use-after-free in win_close()

Problem:  [security]: Use-after-free in win_close()
Solution: Check window is valid, before accessing it

If the current window structure is no longer valid (because a previous
autocommand has already freed this window), fail and return before
attempting to set win->w_closing variable.

Add a test to trigger ASAN in CI

25aabc2b8e

Co-authored-by: Christian Brabandt <cb@256bit.org>

src/nvim/window.c

@@ -2661,6 +2661,9 @@ int win_close(win_T *win, bool free_buf, bool force)
       reset_VIsual_and_resel();  // stop Visual mode
 
       other_buffer = true;
+      if (!win_valid(win)) {
+        return FAIL;
+      }
       win->w_closing = true;
       apply_autocmds(EVENT_BUFLEAVE, NULL, NULL, false, curbuf);
       if (!win_valid(win)) {