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.
diff --git a/src/testdir/Make_all.mak b/src/testdir/Make_all.mak
index c01634c..bb646ee 100644
--- a/src/testdir/Make_all.mak
+++ b/src/testdir/Make_all.mak
@@ -164,6 +164,7 @@
 	    test_job_fails.res \
 	    test_json.res \
 	    test_langmap.res \
+	    test_lua.res \
 	    test_man.res \
 	    test_marks.res \
 	    test_matchadd_conceal.res \
@@ -172,8 +173,8 @@
 	    test_nested_function.res \
 	    test_netbeans.res \
 	    test_normal.res \
-	    test_paste.res \
 	    test_packadd.res \
+	    test_paste.res \
 	    test_perl.res \
 	    test_profile.res \
 	    test_python2.res \
diff --git a/src/testdir/test_lua.vim b/src/testdir/test_lua.vim
new file mode 100644
index 0000000..944ad90
--- /dev/null
+++ b/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