fix(filetype): handle invalid bufnr in _getlines(), _getline() #36272

**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()`.

runtime/lua/vim/filetype.lua

@@ -46,6 +46,10 @@ end
 ---@param end_lnum integer|nil The line number of the last line (inclusive, 1-based)
 ---@return string[] # Array of lines
 function M._getlines(bufnr, start_lnum, end_lnum)
+  if not bufnr or bufnr < 0 then
+    return {}
+  end
+
   if start_lnum then
     return api.nvim_buf_get_lines(bufnr, start_lnum - 1, end_lnum or start_lnum, false)
   end
@@ -59,6 +63,10 @@ end
 ---@param start_lnum integer The line number of the first line (inclusive, 1-based)
 ---@return string
 function M._getline(bufnr, start_lnum)
+  if not bufnr or bufnr < 0 then
+    return ''
+  end
+
   -- Return a single line
   return api.nvim_buf_get_lines(bufnr, start_lnum - 1, start_lnum, false)[1] or ''
 end