**Problem:**
`vim.filetype.match({ filename = 'a.sh' })` returns `nil` because
an invalid buffer ID is passed to `vim.api.nvim_buf_get_lines()`.
For filetypes like `csh`, `txt`, or any other extensions that call
`_getlines()` or `_getline()` to detect their filetypes, the same
issue occurs.
When only the `filename` argument is passed, an error is raised
inside a `pcall()` that wraps the filetype detection function,
causing it to return no value without showing any error message.
**Solution:**
Validate the `bufnr` value in `_getlines()` and `_getline()`.
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