Sort of revert linked.h changes, seeing as it does not work well :(

app/gimage.c

@@ -1351,6 +1351,8 @@ gimage_get_layer_index (GImage *gimage, int layer_ID)
   link_ptr layers = gimage->layers;
   int index = 0;
 
+  g_return_val_if_fail(gimage != NULL, -1);
+
   while (layers)
     {
       layer = (Layer *) layers->data;

app/linked.h

@@ -15,17 +15,34 @@
  * along with this program; if not, write to the Free Software
  * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
  */
-
-/* This is compatibility stuff only... */
 #ifndef __LINKED_H__
 #define __LINKED_H__
 
+#ifndef USE_GSLIST_VERSION
+typedef struct _link
+{
+  void *data;
+  struct _link *next;
+} *link_ptr;
+
+extern link_ptr alloc_list (void);
+extern link_ptr free_list (link_ptr);
+extern link_ptr add_to_list (link_ptr, void *);
+extern link_ptr append_to_list (link_ptr, void *);
+extern link_ptr insert_in_list (link_ptr, void *, int);
+extern link_ptr remove_from_list (link_ptr, void *);
+extern link_ptr next_item (link_ptr);
+extern link_ptr nth_item (link_ptr, int);
+extern int      list_length (link_ptr);
+
+#else /* USE_GSLIST_VERSION */
+
 #include <glib.h>
 
 typedef GSList * link_ptr;
 
 #define alloc_list() g_slist_alloc()
-#define free_list(x) g_slist_free((x))
+#define free_list(x) g_slist_free((x)), NULL
 #define add_to_list(x, y) g_slist_prepend((x), (y))
 #define append_to_list(x, y) g_slist_append((x), (y))
 #define insert_in_list(x, y, z) g_slist_insert((x), (y), (z))
@@ -34,4 +51,6 @@ typedef GSList * link_ptr;
 #define nth_item(x, y) g_slist_nth((x), (y))
 #define list_length(x) g_slist_length((x))
 
+#endif /* USE_GSLIST_VERSION */
+
 #endif