Remove redundant checks for the empty overlay tree

Remove redundant checks for the empty overlay tree

* src/alloc.c (mark_buffer): Remove b->overlays check.
* src/buffer.c (copy_overlays): Also.
* src/buffer.c (swap_buffer_overlays, overlays_in): Also.
  (next_overlay_change, previous_overlay_change): Also.
  (overlay_touches_p, Foverlay_lists): Also.
  (evaporate_overlays): Also.
* src/xdisp.c (load_overlay_strings): Also.