desktop.c: don't print errors when cursor is on resize-indicator

desktop.c: don't print errors when cursor is on resize-indicator

In f347a81, I thought there are only window contents and SSD under
`view->scene_tree` and forgot about the resize-indicator.

I also refactored the logic around it:
- Remove `ret.node->type == WLR_SCENE_NODE_BUFFER` check since it's
  already done by `lab_wlr_surface_from_node()`
- Eliminate duplicated call to `lab_wlr_surface_from_node()`