fix(window): avoid UAF in close_windows (#40853)

test/functional/api/buffer_spec.lua

@@ -2503,6 +2503,20 @@ describe('api/buf', function()
       ok(not api.nvim_buf_is_loaded(b))
       ok(api.nvim_buf_is_valid(b))
     end)
+
+    it('does not crash if WinClosed closes the next tabpage #40852', function()
+      exec_lua(function()
+        vim.cmd.tabnew()
+        vim.api.nvim_create_autocmd('WinClosed', {
+          callback = function()
+            pcall(vim.api.nvim_win_close, 0, true)
+          end,
+        })
+        local bufs = vim.api.nvim_list_bufs()
+        vim.api.nvim_buf_delete(bufs[#bufs - 1], { force = true })
+      end)
+      assert_alive()
+    end)
   end)
 
   describe('nvim_buf_get_mark', function()