libview: Check number of pages on ev_view_document_changed_cb

Fix crash on corrupted PDF with no pages.

Thanks to Juha Kylmänen from OUSPG who were testing robutness.
(CVE-2013-3718).

libview: Fix warning on ev_view_document_changed_cb
There was an introduction of a warning when fixing the bug
https://bugzilla.gnome.org/show_bug.cgi?id=701302

If ev_view_document_changed_cb () is emitted when there
is no previous document, the check fails.

This patch change the order of the condition checks.

origin commits:
https://git.gnome.org/browse/evince/commit/?h=gnome-3-8&id=62b1585
https://git.gnome.org/browse/evince/commit/?h=gnome-3-8&id=5431595

libview/ev-view.c

@@ -6769,12 +6769,15 @@ ev_view_document_changed_cb (EvDocumentModel *model,
 			g_object_unref (view->document);
                 }
 
-		view->document = document;
+		view->document = document ? g_object_ref (document) : NULL;
 		view->find_result = 0;
 
 		if (view->document) {
+			if (ev_document_get_n_pages (view->document) <= 0 ||
+			    !ev_document_check_dimensions (view->document))
+				return;
+
 			ev_view_set_loading (view, FALSE);
-			g_object_ref (view->document);
 			setup_caches (view);
                 }