patch 9.0.1763: crash when passing invalid buffer to undotree()

Problem:  crash when passing invalid buffer to undotree()
Solution: Use get_buf_arg() instead of tv_get_buf_from_arg().

closes: #12862
closes: #12830

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: zeertzjq <zeertzjq@outlook.com>

src/testdir/test_undo.vim

@@ -134,6 +134,18 @@ func Test_undotree_bufnr()
   call assert_notequal(d1, d)
   call assert_equal(d2, d)
 
+  " error cases
+  call assert_fails('call undotree(-1)', 'E158:')
+  call assert_fails('call undotree("nosuchbuf")', 'E158:')
+
+  " after creating a buffer nosuchbuf, undotree('nosuchbuf') should
+  " not error out
+  new nosuchbuf
+  let d = {'seq_last': 0, 'entries': [], 'time_cur': 0, 'save_last': 0, 'synced': 1, 'save_cur': 0, 'seq_cur': 0}
+  call assert_equal(d, undotree("nosuchbuf"))
+  " clean up
+  bw nosuchbuf
+
   " Drop created windows
   set ul&
   new

src/undo.c

@@ -3726,23 +3726,18 @@ u_undofile_reset_and_delete(buf_T *buf)
     void
 f_undotree(typval_T *argvars UNUSED, typval_T *rettv)
 {
-    typval_T	*tv = &argvars[0];
-    buf_T	*buf;
-    dict_T	*dict;
-    list_T	*list;
-
     if (in_vim9script() && check_for_opt_buffer_arg(argvars, 0) == FAIL)
 	return;
 
-    if (tv->v_type == VAR_UNKNOWN)
-	buf = curbuf;
-    else
-	buf = tv_get_buf_from_arg(tv);
-
     if (rettv_dict_alloc(rettv) == FAIL)
 	return;
 
-    dict = rettv->vval.v_dict;
+    typval_T	*tv = &argvars[0];
+    buf_T	*buf = tv->v_type == VAR_UNKNOWN ? curbuf : get_buf_arg(tv);
+    if (buf == NULL)
+	return;
+
+    dict_T *dict = rettv->vval.v_dict;
 
     dict_add_number(dict, "synced", (long)buf->b_u_synced);
     dict_add_number(dict, "seq_last", buf->b_u_seq_last);
@@ -3751,7 +3746,7 @@ f_undotree(typval_T *argvars UNUSED, typval_T *rettv)
     dict_add_number(dict, "time_cur", (long)buf->b_u_time_cur);
     dict_add_number(dict, "save_cur", buf->b_u_save_nr_cur);
 
-    list = list_alloc();
+    list_T *list = list_alloc();
     if (list != NULL)
     {
 	u_eval_tree(buf, buf->b_u_oldhead, list);

src/version.c

@@ -695,6 +695,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1763,
 /**/
     1762,
 /**/