Fix new linting issues

app/channel.go

@@ -1800,7 +1800,7 @@ func (a *App) MarkChannelAsUnreadFromPost(postID string, userID string) (*model.
 	}
 
 	channelUnread, updateErr := a.Srv.Store.Channel().UpdateLastViewedAtPost(post, userID, unreadMentions)
-	if err != nil {
+	if updateErr != nil {
 		return channelUnread, updateErr
 	}
 

app/post_test.go

@@ -1276,6 +1276,7 @@ func TestCountMentionsFromPost(t *testing.T) {
 			ChannelId: channel.Id,
 			Message:   "test",
 		}, channel, false)
+		require.Nil(t, err)
 
 		count, err := th.App.countMentionsFromPost(user2, post1)