magit-insert-section--finish: Fix delayed marker creation

Fix regression added in [1: 153cba3].  We are mapping over
children; using the bounds of the parent was not intended.

1: 2024-05-04 153cba3
   magit-insert-section--finish: Avoid some oref calls

lisp/magit-section.el

@@ -1466,9 +1466,10 @@ anything this time around.
     (cond ((eq obj magit-root-section)
            (when (eq magit-section-inhibit-markers 'delay)
              (setq magit-section-inhibit-markers nil)
-             (magit-map-sections (lambda (section)
-                                   (oset section start (copy-marker beg t))
-                                   (oset section end   (copy-marker end t)))))
+             (magit-map-sections
+              (lambda (section)
+                (oset section start (copy-marker (oref section start) t))
+                (oset section end   (copy-marker (oref section end)   t)))))
            (let ((magit-section-cache-visibility nil))
              (magit-section-show obj)))
           (magit-section-insert-in-reverse