fix(vim.hl): nvim_buf_del_extmark on invalid buffer #33331

Problem:
nvim_buf_del_extmark error if buffer is destroyed before timer stops

Solution:
check nvim_buf_is_valid.

test/functional/lua/hl_spec.lua

@@ -222,6 +222,7 @@ describe('vim.hl.on_yank', function()
 
   it('does not show errors even if buffer is wiped before timeout', function()
     command('new')
+    n.feed('ifoo<esc>') -- set '[, ']
     exec_lua(function()
       vim.hl.on_yank({
         timeout = 10,