Check if the buffer is valid when trying to attach TS

[ollien]

I found a small race condition in the TS install path, where a closed buffer would pop an error after the buffer isntalled. You can repro by running the below script after deleting your zig queries/parsers from the state directory and running nvim -u repro.lua.

dofile(vim.fn.expand("/path/to/kickstart.nvim/init.lua"))

vim.fn.mkdir("/tmp/ts-repro", "p")
vim.fn.writefile({ "const x = 1;" }, "/tmp/ts-repro/sample.zig")

vim.schedule(function()
	vim.cmd("edit /tmp/ts-repro/sample.zig")
	vim.cmd("bwipeout!")
end)

You should expect to see

vim.schedule callback: /opt/nvim/share/nvim/runtime/lua/vim/treesitter.lua:443:
 Invalid buffer id: 1   

This PR fixes that bug by checking if the buffer is still valid before attempting to attach TS.

[oriori1703]

Thanks for the great reproduction and fix :)