fix post editing when channel links are present (#7938)

app/post.go

@@ -207,7 +207,7 @@ func (a *App) FillInPostProps(post *model.Post, channel *model.Channel) *model.A
 	if len(channelMentions) > 0 {
 		if channel == nil {
 			result := <-a.Srv.Store.Channel().GetForPost(post.Id)
-			if result.Err == nil {
+			if result.Err != nil {
 				return model.NewAppError("FillInPostProps", "api.context.invalid_param.app_error", map[string]interface{}{"Name": "post.channel_id"}, result.Err.Error(), http.StatusBadRequest)
 			}
 			channel = result.Data.(*model.Channel)

app/post_test.go

@@ -175,4 +175,13 @@ func TestPostChannelMentions(t *testing.T) {
 			"display_name": "Mention Test",
 		},
 	}, result.Props["channel_mentions"])
+
+	post.Message = fmt.Sprintf("goodbye, ~%v!", channelToMention.Name)
+	result, err = th.App.UpdatePost(post, false)
+	require.Nil(t, err)
+	assert.Equal(t, map[string]interface{}{
+		"mention-test": map[string]interface{}{
+			"display_name": "Mention Test",
+		},
+	}, result.Props["channel_mentions"])
 }