Avoid internal errors with setloclist()

All syntastic users experienced this problem:

  E685: Internal error: get_tv_string_buf()

It's reproducable with:

  :call setloclist(0, [''])

So, not given optional arguments to setloclist() lead to some fields not
inizilied and the code took the wrong branches.

src/nvim/eval.c

@@ -15251,7 +15251,7 @@ static void set_qf_ll_list(win_T *wp, typval_T *list_arg, typval_T *action_arg,
         action = *act;
     }
 
-    if (title_arg->v_type != VAR_UNKNOWN) {
+    if (title_arg->v_type == VAR_STRING) {
       title = get_tv_string_chk(title_arg);
       if (!title) {
         return;  // type error; errmsg already given