Backport the implementation of gtk_list_store_iter_is_valid from gtk+-3.

Index: gtk+-2.24.33/gtk/gtkliststore.c
===================================================================
--- gtk+-2.24.33.orig/gtk/gtkliststore.c
+++ gtk+-2.24.33/gtk/gtkliststore.c
@@ -1195,16 +1195,31 @@ gboolean
 gtk_list_store_iter_is_valid (GtkListStore *list_store,
                               GtkTreeIter  *iter)
 {
+  GSequenceIter *seq_iter;
+
   g_return_val_if_fail (GTK_IS_LIST_STORE (list_store), FALSE);
   g_return_val_if_fail (iter != NULL, FALSE);
 
-  if (!VALID_ITER (iter, list_store))
-    return FALSE;
+  /* can't use VALID_ITER() here, because iter might point
+   * to random memory.
+   *
+   * We MUST NOT dereference it.
+   */
 
-  if (g_sequence_iter_get_sequence (iter->user_data) != list_store->seq)
+  if (iter == NULL ||
+      iter->user_data == NULL ||
+      list_store->stamp != iter->stamp)
     return FALSE;
 
-  return TRUE;
+  for (seq_iter = g_sequence_get_begin_iter (list_store->seq);
+       !g_sequence_iter_is_end (seq_iter);
+       seq_iter = g_sequence_iter_next (seq_iter))
+    {
+      if (seq_iter == iter->user_data)
+        return TRUE;
+    }
+
+  return FALSE;
 }
 
 static gboolean real_gtk_list_store_row_draggable (GtkTreeDragSource *drag_source,
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<tr><th>author</th><td>Efraim Flashner &lt;efraim@flashner.co.il&gt;</td><td class='right'>2024-04-02 12:19:27 +0300</td></tr>
<tr><th>committer</th><td>Efraim Flashner &lt;efraim@flashner.co.il&gt;</td><td class='right'>2024-07-19 00:46:51 +0300</td></tr>
<tr><th>commit</th><td colspan='2' class='oid'><a href='/guix/commit/gnu/packages?id=864bf7cc7388b39958d65bd7066a7e2423e7f3a2'>864bf7cc7388b39958d65bd7066a7e2423e7f3a2</a> (<a href='/guix/patch/gnu/packages?id=864bf7cc7388b39958d65bd7066a7e2423e7f3a2'>patch</a>)</td></tr>
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<div class='commit-subject'>gnu: rust-phf-macros-0.9: Downgrade to 0.9.0.</div><div class='commit-msg'>Later versions of this crate are yanked by upstream.

* gnu/packages/crates-io.scm (rust-phf-macros-0.9): Downgrade to 0.9.0.
[cargo-development-inputs]: Replace rust-phf-0.9 with 0.8.

Change-Id: I0fd5e2bf0e0c4ff32279a6b0359a3eb1dbf1c9ff
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