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/Makefile

@@ -2147,6 +2147,7 @@ test_arglist \
 	test_langmap \
 	test_largefile \
 	test_lispwords \
+	test_lua \
 	test_man \
 	test_mapping \
 	test_marks \

src/if_lua.c

@@ -1716,6 +1716,8 @@ ex_luado(exarg_T *eap)
     const char *s = (const char *) eap->arg;
     luaL_Buffer b;
     size_t len;
+    buf_T *was_curbuf = curbuf;
+
     if (lua_init() == FAIL) return;
     if (u_save(eap->line1 - 1, eap->line2 + 1) == FAIL)
     {
@@ -1739,6 +1741,10 @@ ex_luado(exarg_T *eap)
     lua_replace(L, -2); /* function -> body */
     for (l = eap->line1; l <= eap->line2; l++)
     {
+	/* Check the line number, the command my have deleted lines. */
+	if (l > curbuf->b_ml.ml_line_count)
+	    break;
+
 	lua_pushvalue(L, -1); /* function */
 	luaV_pushline(L, curbuf, l); /* current line as arg */
 	lua_pushinteger(L, l); /* current line number as arg */
@@ -1747,6 +1753,9 @@ ex_luado(exarg_T *eap)
 	    luaV_emsg(L);
 	    break;
 	}
+	/* Catch the command switching to another buffer. */
+	if (curbuf != was_curbuf)
+	    break;
 	if (lua_isstring(L, -1)) /* update line? */
 	{
 #ifdef HAVE_SANDBOX

src/testdir/Make_all.mak

@@ -164,6 +164,7 @@ NEW_TESTS = test_arglist.res \
 	    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 @@ NEW_TESTS = test_arglist.res \
 	    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 \

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

src/version.c

@@ -764,6 +764,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    268,
 /**/
     267,
 /**/