patch 8.0.0268: may get ml_get error when :luado deletes lines

Problem:    May get ml_get error when :luado deletes lines or switches to
            another buffer. (Nikolai Pavlov, issue #1421)
Solution:   Check the buffer and line every time.

src/testdir/test_lua.vim

@@ -0,0 +1,22 @@
+" Tests for Lua.
+" TODO: move tests from test85.in here.
+
+if !has('lua')
+  finish
+endif
+
+func Test_luado()
+  new
+  call setline(1, ['one', 'two', 'three'])
+  luado vim.command("%d_")
+  bwipe!
+
+  " Check switching to another buffer does not trigger ml_get error.
+  new
+  let wincount = winnr('$')
+  call setline(1, ['one', 'two', 'three'])
+  luado vim.command("new")
+  call assert_equal(wincount + 1, winnr('$'))
+  bwipe!
+  bwipe!
+endfunc