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,
2ebd7e7be683c07241bbd8a91919'>tree</a><a href='/guix/commit/gnu/services/science.scm?id=d2111894c48b2ebd7e7be683c07241bbd8a91919'>commit</a><a href='/guix/diff/gnu/services/science.scm?id=d2111894c48b2ebd7e7be683c07241bbd8a91919'>diff</a></td><td class='form'><form class='right' method='get' action='/guix/log/gnu/services/science.scm'>
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<div class='path'>path: <a href='/guix/log/?id=d2111894c48b2ebd7e7be683c07241bbd8a91919'>root</a>/<a href='/guix/log/gnu?id=d2111894c48b2ebd7e7be683c07241bbd8a91919'>gnu</a>/<a href='/guix/log/gnu/services?id=d2111894c48b2ebd7e7be683c07241bbd8a91919'>services</a>/<a href='/guix/log/gnu/services/science.scm?id=d2111894c48b2ebd7e7be683c07241bbd8a91919'>science.scm</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/gnu/services/science.scm?id=d2111894c48b2ebd7e7be683c07241bbd8a91919&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2021-11-30 01:08:55 +0100'>2021-11-30</span></td><td><a href='/guix/commit/gnu/services/science.scm?id=892f1b7273d57b25940700877a02618fe826cc08'>services: Accept &lt;inferior-package&gt;s in lieu of &lt;package&gt;s.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/services/authentication.scm (fprintd-configuration)
(nslcd-configuration): Substitute file-like objects for package ones.
* gnu/services/cgit.scm (cgit-configuration, opaque-cgit-configuration):
Likewise.
* gnu/services/cups.scm (package-list?, cups-configuration): Likewise.
* gnu/services/dns.scm (verify-knot-configuration)
(ddclient-configuration): Likewise.
* gnu/services/docker.scm (docker-configuration): Likewise.
* gnu/services/file-sharing.scm (transmission-daemon-configuration): Likewise.
* gnu/services/getmail.scm (getmail-configuration): Likewise.
* gnu/services/mail.scm (dovecot-configuration)
(opaque-dovecot-configuration): Likewise.
* gnu/services/messaging.scm (prosody-configuration)
(opaque-prosody-configuration): Likewise.
* gnu/services/monitoring.scm (zabbix-server-configuration)
(zabbix-agent-configuration): Likewise.
* gnu/services/networking.scm (opendht-configuration): Likewise.
* gnu/services/pm.scm (tlp-configuration): Likewise.
* gnu/services/telephony.scm (jami-configuration): Likewise.
* gnu/services/virtualization.scm (libvirt-configuration)
(qemu-guest-agent-configuration): Likewise.
* gnu/services/vpn.scm (openvpn-client-configuration): Likewise.
</span></span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2020-12-09 12:55:02 +0200'>2020-12-09</span></td><td><a href='/guix/commit/gnu/services/science.scm?id=533935ccf71bef93f2f146fba1b2bb6402559965'>services: science.scm: Add missing copyright headers.</a><span class='msg-avail'>...<span class='msg-tooltip'>This module was always supposed to have the GPL3 header in place.
Follow-up to 8f19e63f760fe02f9969225da2f3f3c36801218f.

* gnu/services/science.scm: Add Copyright headers.
</span></span></td><td>Efraim Flashner</td></tr>