patch 8.0.0270: may get ml_get error when :rubydo deletes lines

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

src/if_ruby.c

@@ -783,6 +783,7 @@ void ex_rubydo(exarg_T *eap)
 {
     int state;
     linenr_T i;
+    buf_T   *was_curbuf = curbuf;
 
     if (ensure_ruby_initialized())
     {
@@ -792,6 +793,8 @@ void ex_rubydo(exarg_T *eap)
 	{
 	    VALUE line;
 
+	    if (i > curbuf->b_ml.ml_line_count)
+		break;
 	    line = vim_str2rb_enc_str((char *)ml_get(i));
 	    rb_lastline_set(line);
 	    eval_enc_string_protect((char *) eap->arg, &state);
@@ -800,6 +803,8 @@ void ex_rubydo(exarg_T *eap)
 		error_print(state);
 		break;
 	    }
+	    if (was_curbuf != curbuf)
+		break;
 	    line = rb_lastline_get();
 	    if (!NIL_P(line))
 	    {

src/testdir/test_ruby.vim

@@ -32,3 +32,20 @@ func Test_ruby_evaluate_dict()
   redir END
   call assert_equal(['{"a"=>"foo", "b"=>123}'], split(l:out, "\n"))
 endfunc
+
+func Test_rubydo()
+  " Check deleting lines does not trigger ml_get error.
+  new
+  call setline(1, ['one', 'two', 'three'])
+  rubydo Vim.command("%d_")
+  bwipe!
+
+  " Check switching to another buffer does not trigger ml_get error.
+  new
+  let wincount = winnr('$')
+  call setline(1, ['one', 'two', 'three'])
+  rubydo Vim.command("new")
+  call assert_equal(wincount + 1, winnr('$'))
+  bwipe!
+  bwipe!
+endfunc

src/version.c

@@ -764,6 +764,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    270,
 /**/
     269,
 /**/