Steps to reproduce:

1. setting `vim.highlight.on_yank`
   ```
   vim.api.nvim_create_autocmd({ "TextYankPost" }, {
     pattern = { "*" },
     callback = function()
       vim.highlight.on_yank({ timeout = 200 })
     end,
   })
   ```
2. repeat typing `yeye` ...
3. causes the following error.
   ```
   Error executing vim.schedule lua callback: vim/_editor.lua:0: handle 0x01e96970 is already closing
   stack traceback:
           [C]: in function 'close'
           vim/_editor.lua: in function ''
           vim/_editor.lua: in function <vim/_editor.lua:0>
   ```

📝 Test result before fix:

    [----------] Global test environment setup.
    [----------] Running tests from test/functional/lua/highlight_spec.lua
    [ RUN      ] vim.highlight.on_yank does not show errors even if buffer is wiped before timeout: 15.07 ms OK
    [ RUN      ] vim.highlight.on_yank does not show errors even if executed between timeout and clearing highlight: 15.07 ms ERR
    test/helpers.lua:73: Expected objects to be the same.
    Passed in:
    (string) 'Error executing vim.schedule lua callback: vim/_editor.lua:0: handle 0x02025260 is already closing
    stack traceback:
            [C]: in function 'close'
            vim/_editor.lua: in function ''
            vim/_editor.lua: in function <vim/_editor.lua:0>'
    Expected:
    (string) ''

The test added in 274a3504a790a799b28ee89c75e29fb4dbdff41f
does not fail if the code changes are reverted.

Resolve an issue with deferred clearing of highlight failing if the 
buffer is deleted before the timeout by checking whether the
buffer is valid first.