[goffice] Make go_doc_image_fetch more secure.

[Jean Bréfort <jbrefort src gnome org>]

commit 9e78b87a7df2516fb4171c8c8fdfbde4dbe4a2c9
Author: Jean Brefort <jean brefort normalesup org>
Date:   Fri Jul 5 15:51:21 2013 +0200

    Make go_doc_image_fetch more secure.

 ChangeLog            |    4 ++++
 goffice/app/go-doc.c |    7 +++++++
 2 files changed, 11 insertions(+), 0 deletions(-)
---
diff --git a/ChangeLog b/ChangeLog
index ba11229..2a5279d 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+2013-07-05  Jean Brefort  <jean brefort normalesup org>
+
+       * goffice/app/go-doc.c (go_doc_image_fetch): make it more secure.
+
 2013-06-25  Morten Welinder <terra gnome org>
 
        * configure.ac: Post-release bump.
diff --git a/goffice/app/go-doc.c b/goffice/app/go-doc.c
index 601dec8..c7161c4 100644
--- a/goffice/app/go-doc.c
+++ b/goffice/app/go-doc.c
@@ -673,7 +673,14 @@ go_doc_image_fetch (GODoc *doc, char const *id, GType type)
        g_return_val_if_fail (doc && doc->priv->imagebuf, NULL);
        image = g_hash_table_lookup (doc->priv->imagebuf, id);
        if (!image) {
+               g_return_val_if_fail (type != 0, NULL);
                image = g_object_new (type, NULL);
+               if (!GO_IS_IMAGE (image)) {
+                       if (image)
+                               g_object_unref (image);
+                       g_critical ("Invalid image type");
+                       return NULL;
+               }
                go_image_set_name (image, id);
                g_hash_table_replace (doc->priv->imagebuf,
                                      g_strdup (go_image_get_name (image)),