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,
8163212d944705a157210d92'>tree</a><a href='/guix/commit/etc/guix-daemon.conf.in?id=85a50e9b828e82638163212d944705a157210d92'>commit</a><a href='/guix/diff/etc/guix-daemon.conf.in?id=85a50e9b828e82638163212d944705a157210d92'>diff</a></td><td class='form'><form class='right' method='get' action='/guix/log/etc/guix-daemon.conf.in'>
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<div class='path'>path: <a href='/guix/log/?id=85a50e9b828e82638163212d944705a157210d92'>root</a>/<a href='/guix/log/etc?id=85a50e9b828e82638163212d944705a157210d92'>etc</a>/<a href='/guix/log/etc/guix-daemon.conf.in?id=85a50e9b828e82638163212d944705a157210d92'>guix-daemon.conf.in</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/etc/guix-daemon.conf.in?id=85a50e9b828e82638163212d944705a157210d92&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2021-07-07 23:54:20 -0400'>2021-07-07</span></td><td><a href='/guix/commit/etc/guix-daemon.conf.in?id=4cbe0127a668cbcc64087c5d311308b59d39b104'>guix-install.sh: Prompt for configuring substitutes discovery.</a><span class='msg-avail'>...<span class='msg-tooltip'>Also ensure prompt_yes_no always print the message with a trailing space,
which is more pleasing to the eye.

* etc/guix-daemon.conf.in &lt;--discover=no&gt;: New guix-daemon option.
* etc/guix-daemon.service.in: Likewise.
* etc/init.d/guix-daemon.in: Likewise.
* etc/openrc/guix-daemon.in: Likewise.
* etc/guix-install.sh (configure_substitute_discovery): New procedure.
(sys_enable_guix_daemon): Ask the user whether automatic substitute discovery
should be enabled.  Set the '--discover' argument accordingly.
(prompt_yes_no): Add a trailing space to the message.
(sys_authorize_build_farms): Remove trailing space from the message argument.
* NEWS (Distribution): Add news.
</span></span></td><td>Maxim Cournoyer</td></tr>