fix permission app error with updatePost (#6749)

api4/post.go

@@ -302,6 +302,10 @@ func searchPosts(c *Context, w http.ResponseWriter, r *http.Request) {
 
 func updatePost(c *Context, w http.ResponseWriter, r *http.Request) {
 	c.RequirePostId()
+	if c.Err != nil {
+		return
+	}
+
 	post := model.PostFromJson(r.Body)
 
 	if post == nil {

api4/post_test.go

@@ -137,6 +137,8 @@ func TestUpdatePost(t *testing.T) {
 
 	msg := "zz" + model.NewId() + " update post"
 	rpost.Message = msg
+	rpost.UserId = ""
+
 	rupost, resp := Client.UpdatePost(rpost.Id, rpost)
 	CheckNoError(t, resp)