fix(editorconfig): check that buffer is valid

Fixes: https://github.com/neovim/neovim/issues/23921
(cherry picked from commit 1cf3184f95)

test/functional/plugin/editorconfig_spec.lua

@@ -6,6 +6,7 @@ local pathsep = helpers.get_pathsep()
 local curbufmeths = helpers.curbufmeths
 local funcs = helpers.funcs
 local meths = helpers.meths
+local exec_lua = helpers.exec_lua
 
 local testdir = 'Xtest-editorconfig'
 
@@ -207,4 +208,15 @@ But not this one
     test_case('3_space.txt', { shiftwidth = 42 })
     test_case('4_space.py', { shiftwidth = 4 })
   end)
+
+  it('does not operate on invalid buffers', function()
+    local ok, err = unpack(exec_lua([[
+      vim.cmd.edit('test.txt')
+      local bufnr = vim.api.nvim_get_current_buf()
+      vim.cmd.bwipeout(bufnr)
+      return {pcall(require('editorconfig').config, bufnr)}
+    ]]))
+
+    eq(true, ok, err)
+  end)
 end)